Can sharks last in unsalted water?

There are some sharks that can live in freshwater environments, but there aren't a lot that can cross over and live in both. The Bull shark is one that has been shown to have the ability to live in both saltwater and freshwater. Interestingly, they have to produce 20 times more urine when living in freshwater.

(See the related link for more about Bull Sharks' freshwater tolerance.)