Dear Fiberbabe: Sharks developed at a time or place where the water was not saline. When the water began to become salty, they began to retain so much urea in their body fluids that they were relatively isotonic with sea water. If you are interested here is a URL about it

http://www.science.mcmaster.ca/Biology/4S03/dm.html

You have to scroll down to D, where it talks about elasmobranchs (which includes sharks)