Actualy fresh water is used to treat parasitic desiases in sea turtles Most sea turts tolerate fresh water in the short term. There is even rumors that some have been kept this way for years in roadside zoos But their health would be probably compromised specialy their kidneys, but really the true awnser is that this question is pretty much unawsered. Could they adapt to live in hard fresh water? Very likely. They arent as delicate as marine fish for that matter
I think it helps that they breath air since their body wouldn't need to filter out salt it shouldn't be too bard to adapt to fresh water. I think it's similar to dolphins. I do believe they found bottlenose dolphins in a river. Unless I am wrong but once you get past the estuary it's pretty much freshwater.
I seem to recall reading in an old "turtle care" type book (how old? it discussed sea turtle captive care for one) that sea turts kept in fresh water are highly susceptible to fungal infections.
Tell you what, I'll hit the beach here in Daytona, catch one, and try it (shh don't tell). If it works one of you guys can trade me a black market asian aro for my freshwater hawksbill.
I agree with Coura in that it would be very hard on the turtle's kidneys. Aquatic animals have evolved an osmotic balance that allows them to keep their internal salinity constant whether it be by retaining (freshwater) or excreting (marine) salt.
I think the turtle would be fine in the short term, but it would certainly be a very stressful situation and I doubt the turtle would thrive. The fact that they are open-ocean dwellers (as opposed to living in bays/estuaries) would make their tolerance for salinity changes even less.