In a future world, where there is almost no fresh water resources left, a radical scientist comes up with an idea to turn all the oceans into fresh water.

The general idea is to be able to precipitate the salt, so that it sinks to the ocean bed. Whatever sediment and salt get added through rivers (of which, there are very few) and drains, immediately precipitate to the bed, leaving the sea water fresh.

Now, with fresh water in the seas, what things should be considered to maintain the ecological balance as much as possible? We've already lost a lot of animals and ecosystems, we wouldn't want to lose more. Whether or not there are solutions, what are the problems to be considered? (Solutions are welcome!)

One thing I can think of, is the salt water fish. As a solution, the scientist could probably genetically alter them to adapt to fresh water. What else?