Hmmm....the only thing I know related to your query without googling is that salinity is serious business. I know for fish, if a freshwater fish goes to saltwater, it's basically getting slowly turned into fish jerky. If a salty fish goes to freshwater, I guess it's the opposite, the fish would possibly have to fight taking on too much water.
I do know of a story in Nicaragua I think, a small sea was cut off from the ocean and slowly turned to fresh water, and within this new lake were bull sharks that didn't die, and instead became freshwater sharks. So now you can swim in a lake in Nicaragua, or Panama, or somewhere near there, wherever the lake is, and possibly get eaten by a shark.
You think my dad's cool, wait til you see him in a denim vest 🤣🤣🤣.