I’m also interested in the reverse, freshwater to saltwater. Apparently freshwaterism and saltwaterism aren’t the preferred terms. I want to learn more about the physiological changes required for an organism that typically lives in one environment to survive and thrive in the other, but I don’t know what to search for. I’m sure the wikipedia is full of juicy knowledge that will help me procrastinate from cleaning, but I need to know where to look.
I’m not sure of the technical term for the actual evolutionary movement from freshwater to salt, but here’s some jargon to get you started.
The basic problem is that the body fluids of fish, and other vertebrates, are saltier than freshwater, but much more dilute than saltwater. Since fish don’t generally have impermeable skins (and in any case must move water across their gills to breathe), this means that freshwater fish have a problem with losing salt, while saltwater fish have a problem with losing water.
Some fish can survive in a range of salinities. These fish are called euryhaline. The contrasting term is stenohaline, for fish that are confined to a narrow range of salinities (either fresh or salt).
Some fish move between salt and freshwater during their lives. Fish that are hatched in freshwater and move to salt (and then back to fresh to breed), like salmon, are called anadromous. Those that are hatched in saltwater and then move to fresh, like eels, are called catadromous.
Thanks! That’s enough to get me started. I knew the word anadromous; I think I encountered it after cooking some steelhead trout and reading about them. But I didn’t know the others.
As a specific example of one aspect of what Colibri is saying: I used to have several largish-sized (150+ gallons each) coral reef tanks, salinity of normal ocean water, extremely purified/maintained-in-quality, etc. But wife had a liking of a really common freshwater aquaria fish, mollies specifically, and wanted a freshwater tank just for them (no way, I thought, how friggin’ boring!). So instead I appeased her happily and got a dozen or so mollies (really cheap, too) of various ‘styles’/appearances and used an old 10g tank initially for them for acclimation to ‘ocean water’ and slowly, daily, increased salinity of the water to reef-tanks’ level - took about two weeks, iirc. No problems at all - fish healthy whole time and actually brightened up a bit in their coloration, fwiw.
The gang of mollies was released into a tank in which I had no predatory-type of creature(s), just lps/sps corals upon mounds of live-rock and a herd of various hermits/snails for housekeeping of the deep-sand bed bio-filtration and tank walls. No other fish in there at all.
Quite a few friends who came over were astonished to see mollies in a reef tank - and some absolutely would not believe they were ‘real mollies’ as they had the same type fish in their freshwater tanks at their homes.There babies born semi-regularly but very few would last long with the large overflow and sump/protein-skimmer used. The current was pretty strong in that tank. I think I had like 5 or 6 more than original after about a year or so, iirc. Hard to count 'em accurately amongst all the other stuff in tank 
Some (many?) species of fish can acclimate their physiology to accomodate a wide range of salinities, with Colibri’s links being great places to start, as you’ve already seen (on preview)…
Interestingly, it appears that the ancestors of most ray-finned fish originally inhabited freshwater. Most saltwater bony fish had freshwater ancestors.
The relative tolerance of different groups of fishes has been important in determining biogeographical patterns of distribution. Primary freshwater fish are those that have virtually no salt tolerance, and are unable to cross ocean barriers. Their distributions indicate that land masses were once connected. Secondary freshwater fish have some tolerance of saltwater, and are sometimes able to cross saltwater barriers. While technical, this is one of the classic papers on this topic.
I see that I was using anadramous incorrectly for any fish that switches, whereas Atlantic eels are catadramous because they breed in the ocean. Which your earlier link describes, but I thought I was all smartypants already and hadn’t clicked on it.
I had thought this all required very slow (generational) adaptions, but I see that many fish are already equipped for it.
The general term for fish that move between fresh and saltwater is diadromous, with anadromous and catadromous as subcategories (as well as several other mentione in the link).
An interesting case are sticklebacks, which have populations in both saltwater and fresh that look quite different. The freshwater forms have less armor. This form seems to have evolved multiple times independently whenever sticklebacks have colonized freshwater habitats from the sea.
While diving down the internet rabbit hole, I encountered this thread from 2014: Are there insects in the sea? I have nothing to add on the topic, but others might find it interesting.
I was also looking at plants. It looks like there are few marine vascular plants. Angiosperms, even. I couldn’t tell when seagrass entered the sea, but I’m assuming they’re descended at some point from land plants (? Not sure if there is an impetus to develop a vasculature otherwise, but I’m out of my depth), which are descended from (skipping some steps) freshwater green algae, which are descended from marine green algae. That’s quite a large circle.
I didn’t see any discussion of cellular salt levels, and what’s different in these plants that allows them to thrive in a marine environment.