It almost seems like the perfect form of renewable energy. With two tide cycles per day, the amount of energy produced is predictable and consistent. The tides are never going to stop. Most large cities are near coasts, meaning that the electricity produced would not need to be transported very far.
I think it would require enourmous amounts of beach from property to form reservoirs or it would be costly to build giant floats. I think it would be great for small villages on a coast.
I suspect that the requirement to build large machines with moving parts that need to be constantly submerged in salt water presents issues. And then you have to ask what the actual power density is, and how much offshore area you’re willing to dedicate to the project.
In general the energy is predictable but not consistent. The cycle of tides has a complex overlying function with spring and neap tides, and the occasional dodge tide. Plus local weirdness depending upon all sorts of other factors.
You need some sort of useful natural dam to harness the energy, and along with that a usefully large tidal variation. There are not all that many places that have these. Without, it just isn’t economic.
Plus, the environmental factors alone would put the kibosh on most plans.
They’re in the process of installing a massive tidal array in Scotland. The array requires 1.4 square miles of seabed under fast flowing water between an island and the mainland. I suppose ideal locations for economic installations of arrays such as this are few and far between.
Good question and it has puzzled me too.
The core answer is that the marine environment is extremely harsh. I’d guess that there is not a man-made sea structure in the world which has not been damaged by the ocean. Sea-water is corrosive and the force of water in storms is unforgivable.
We have not yet reached the engineering capacity to build cheap strong impervious structures and generators. Oh we can build them but its NASA level spending so the alternative is wave power which is a pale substitute.
True. Plus the Scottish Government for some reason drools over tidal power. We are to be “industry leaders” and so on. I’ll take it all with a pinch of salt until it proves to be an ideal location and a successful project. I suspect it may all become a white elephant.
Another UK tidal power project (though only proposed, and currently abandoned) is teh Severn Barrage:
I love the idea, and think it would be a great example of ‘Muscular Environmentalism’ - what’s the jeopardy of estuary habitats, when we’re saving the World?
Unless you have a large area of strong tidal current (as seems to be the case for the Scottish submerged turbine scheme), a significant factor has to be low power density, which means tidal power stations that produce a useful amount of electricity must be huge and expensive.
In only limited areas of the world is the tidal range more than 2 meters, whereas the range needed for anything resembling efficient output is probably above 7m (power is proportional to the square of the tidal range).
Here are the areas with enough tidal range to make tidal power attractive:
- Bay of Fundy, Canada (has 8 MW installed)
- Bristol Channel and Cardiff Bay, UK
- Normandy, France (has 240 MW installed)
- Magellan Strait, Argentina & Chile
- Cook Inlet, Alaska, USA
- Penzhinskaya Bay, Kamtchatka, Russia
I think the word you wanted there was “unforgiving,” unless you harbor some unique grudge against the ocean. 
Other than that, I think you nailed it. It’s hard enough to build durable fixed structures in salt water like piers and wind turbine bases. Moving machinery is even more difficult. In addition to preventing water intrusion, frequent maintenance will be required in order to eliminate fouling by marine plants, barnacles and mussels.
And then there’s the power density issue. The Hoover Dam is good for a couple thousand megawatts, but that’s predicated on about 700 feet of head and huge volumetric flow rates. Tides generally have a head of just a few feet, and unless you’re actually impounding the tide behind a dam, the flow rate through your turbine will be comparitively small.
Larger pump backs?
If we were to use the tides to produce energy the mechanism would need to create a resistance to the tidal movement. This would increase the drag on the earth’s rotation and before we know it, the earth day is like a billionth of second longer. It that a sacrifice we are willing to make? 
Looks like the Canadian Bay of Fundy tidal project is moving along: http://www.cbc.ca/news/canada/nova-scotia/bay-of-fundy-tidal-power-gets-2-new-entrants-and-4m-1.2590233 . There was one planned in Australia, but i forget where. While not actually tidal, the plan to put turbines in the Gulf Stream off the east coast of Florida died when someone calculated how many whales would get sliced up (i forgot that qty too).
As long as it’s only longer at night.
In a thread about some guy’s idea to clean up the ocean, someone posted a link to this article:
http://inhabitat.com/the-fallacy-of-cleaning-the-gyres-of-plastic-with-a-floating-ocean-cleanup-array/
There’s a bit that deals with ocean power:
“But let’s look at a practical example. My home state of Oregon has been trying to create North America’s first offshore wave energy farm. The first test buoy that was launched, just about 2.5 miles offshore, sank after just a few months. That buoy had a ‘100 year survivability’ rating, and wasn’t just an idea on an Ipad.”
We are slightly confusing each other because there are three main methods to generate electricity from the ocean
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Wave power which harnesses the up and down movement of waves. Scotland seems to be the leader.
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Ocean currents which flow through underwater impellors.
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Tidal power which requires a barrage (dam) and turbines at the entrance to a large estuary.
Number 3 is what I think of when tidal power is mentioned because 1 and 2 do not require tidal flows in order to work.
No love for NW Australia, which has the second largest tidal range in the world?
The experimental projects in the US aren’t very impressive.
Also the effect on the moon, all else being equal, would be to bring it into a lower orbit. Now, I believe that “all else being equal” is not the case, as currently the moon is gradually receding from us. But I don’t want to take any chances. I’ve played majora’s mask.