Just providing living space for potentially billions of more people would serve as a bedrock for future space colonization plans. I don’t think we should NOT colonize space eventually, but I do think that ocean colonization is far more feasible, healthy, ad probable in the modern day than space colonies of the same caliber.
I think the better question is there really a need for either? As others have posted upthread, space colonization is still aspirational fiction, rather than reality, regardless of what several rich white guys think. The technology just isn’t there, and won’t be for decades barring some major breakthrough.
Space based or Mars based research colonies? Even then, that’s a big maybe, but will likely always be small scale based on the cost to lift mass to orbit. Now, if/when we develop better options, then space colonies make more sense, in that any planet has a fixed number of resources, and there are plenty more out there off-planet. But again, decades or centuries are likely to be needed.
So, long term, space has advantages. Short to medium term, certainly not. But again, short to medium term, what do we need from an ocean colony that we cannot get from land? Little to nothing. And the risks are large - we already have substantial political, economic and physical fighting over largely accepted land borders and nations, put something out in international waters, and watch the fighting begin, same for the implausibly engineered subsea colonies.
And with domestic and international terrorism being a thing, the risks are truly mindboggling.
If I was going to want to maximize our usable land, leaving more for less intensive agriculture and wilderness preservation as an example, I’d be building Up rather than at sea. Not a perfect answer either, but the various arcology concepts make more sense than undersea or sea-steading does. Self contained, with mass transportation, recycling, and recoverable energy as the core, build on a large upward scale with a smaller net ground-level footprint than the sprawling cities of the US West for example, seems to be a better overall use of resources.
But again, I’m sure it too would go wrong due to poor planning, maintenance, in-fighting, and everything else humans manage to screw up. Still, would need far fewer major changes in life and technology that sea or space.
The problem with building up is that you would have to tear down centuries worth of existing infrastructure in some places. Meanwhile, making an ocean colony would not suffer from that same logistical issue. An ocean colony would still have far greater logistical issues in its own right, but at least it would not potentially comprise the livelihoods of people already living on land.
This is a very key point. Our own inbred savagery toward one another seems to preclude us from doing advanced things. We need to solve that one as well, before we could consider space and ocean colonies. But again, solving for psychological problems is a lot less sexy than rockets.
I’ve heard Mars ain’t the kind of place to raise your kids.
For one thing, it’s cold as hell.
People have tried for centuries. Unless we literally modify ourselves on a genetic level (and in a sense cease to be fully human during the process) or go full 1984, it ain’t happening.
Plus, the same could be said about literally anything else. Other large projects have been successful in the past regardless of human tribalism. What’s the difference with ocean colonization?
Under the sea. Darling it’s better down where it’s wetter, take it from me.
Most, although certainly not all arcology concepts are built on unused, reclaimed, or megalopolis adjacent lands - such as the epic fail that was the Dongtan / Shanghai plan to build an eco-city outside of Shanghai proper prior to the 2010 World Expo. I have yet to see a discussed arcology (and again, I point at these as MORE practical that sea/space colonies, but despite them being sexy and always coming soon, not a single one is alive and functioning) that would build over an existing infrastructure. No, I half-take that back, I do know there was talk of after the last few times New Orleans was half-wiped off the map to make an ecology on the remnants, but didn’t take it seriously.
There is a third option beyond 1.) other planets/asteroids and 2.) on/under the sea. Underground. Drill straight down anywhere at all. Scoop out some tunnels and rooms. This, except much more.
Seriously - underground has much more merit than on/underwater or in space. It means we can share the vertical space with farming or nature above, and still have access to all the other terrestrial conveniences, and as noted in the link, it’s something practical we are already doing now on a small scale. No need to invest in development of new technology.
The simple and straightforward reason we haven’t done more with oceans is that people know how hard it would be. People have tried to conquer the oceans for millennia and the ocean always wins.
Space will be much harder, true. But nobody’s tried it yet and so space doesn’t cause the visceral reaction to millions of deaths and thousands of shipwrecks that have built up over the centuries. Every country with a coast knows what the ocean will do. Space is an unknown.
Cynically speaking, one just has to look at the Dope. In every thread people enthusiastically post about subjects they obviously know nothing at all about. Space colonization is just an extrapolation of the same desire, except by billionaires and other crackpots. Their big advantage is that by the time they’ll be proved wrong, we’ll all be dead.
Even though suboceanic settlements are still sci-fi, there certainly is a contemporary push towards increased exploitation of seabed resources. Oil rigs have been around for a while, but there is now also serious activity going on towards mining manganese nodules. These are nuggets of (usually) a few inches in size which grow on the seabed as a result of (very slow) sedimentation process, and which are rich in various metals. There has been talk about mining them for decades, but it seems that it is now, after all, picking up pace, and there’s an entire separate international legal regime in place for them, with an International Seabed Authority granting mining concessions and disbursing licence fees among its member states.
As for the imaginative appeal of deep-sea vs space colonisation, I recall that there was, in the 1990s, a Star Trek-style TV show called SeaQuest DSV, about the adventures of a submarine in a world where the ocean bed is heavily colonised. It didn’t last very long.
And on top of that, there’s noone there to raise them if you did.
I dispute this energetically. We are in FQ, so I will simply point out that an average-sized town generates a huge amount of sewage. If you let 100k people crap directly into the water every day, you’re looking at an ecological disaster. A floating colony will need to capture this waste production and either offload it elsewhere or process it thoroughly onsite by some unknown means. I know of no technology that allows this to be handled in a fully self-contained manner on this kind of scale. (Further discussion on this, in particular the likelihood that these people will observe this consideration and not just, y’know, crap into the water, is outside the scope of FQ.)
And speaking of waste, any such colony will need to be self-sufficient economically. A giant bedroom community is not viable; it will need some sort of economic foundation, which means industry, which means more waste, on top of human biology. This, again, will need to be captured and managed, and not just dumped over the side. (And, again, any argumentation as to the odds of compliance are out-of-bounds for this forum.)
The simple practical difficulty of this proposal cannot be understated.
That’s because https://tvtropes.org/pmwiki/pmwiki.php/Main/AtlantisIsBoring
That said, SeaQuest was popular for a time, and, indeed, while it explored plausible underwater scenarios such as conflicts and environmental issues. Ratings dropped when they tried to make it more “star trek in the sea” (yes I checked the Wiki on this, but I had a vague recollection too 
)
By golly you’ve convinced me. Let’s massively waste energy for a few more decades and then terraform Earth! Even with 100F seas it’d have a huge head start over Mars or anywhere else and this way we can shrug and officially plan to continue to kick the can down the road because we all know we’re going to anyway.
There are vast areas of land that are all but empty. Sure, they are frozen wastes or dry deserts but they would be far easier to inhabit than the ocean.
Grossly unfair. @Chronos is not proposing this at all. His point is merely that even in a worst case scenario Earth would remain more hospitable than Mars. His post does not imply we should let that scenario happen.
Ocean colonies would most likely have huge problems with fresh water. It’d have to be transported in, or rain would need to be gathered, or desalination would need to take place (an extremely expensive and energy intense process resulting in the need to dispose of LOTS of hypersaline brine which would contaminate the local environment).
Because it would be extraordinarily expensive, environmentally harmful, and would serve no practical purpose.
I’m guessing you think the world is running out of land. It’s not. Average global population density is 61 people per square kilometre.
Which countries are most densely populated? - Our World in Data.
But assuming that there’s a need for a colony, you’ve got a choice of building an arcology on sparsely populated land, or on open ocean. Advantages of the ocean: Easy access to salt water. Easy potential for fish based aquaculture. Moderate potential for plant/algae based aquaculture based on future technological advancements. Room to put up solar farms and wind farms.
Disadvantages: You have to create a floating structure for your city. That’s going to require a lot of material and far more than a land based equivalent because you need to provide the structure for your city to float, and to maintain position dealing with currents and weather. OK, maybe you go down to the ocean surface. You’ll still have to deal with a surface infrastructure including air pumping plants, energy sources, and any of your aquaculture that requires sunlight. Putting the residencies underwater, doesn’t avoid the fact that aquaculture and green energy rely on wind and sun. And you’re going to have an even greater requirement to keep the above water infrastructure stable. With a floating colony, you’re worried about stability, but will accept drift. With a seabed colony, you really, really want that air supply facility to remain above you.
But okay, let’s just walk past the issues of establishing the colony. You’ve established a floating city in the ocean. City’s have pollution. At a minimum, your proposed oceanic arcology is going to be dumping a huge amount of human sewage into the ocean. You’re also going to be dealing with producing fresh water which leaves brine which is a not very nice pollutant. Also all the waste from the aquaculture. And the emissions from whatever transport your ocean dwelling citizens use to get around. And this utopian idea is assuming that the ocean colony residents won’t be polluting like their land-based cousins. Not a safe assumption, given human nature.