Name: Bull Shark US Locations: Typically in warm, ocean waters, but have been known to swim up some freshwater rivers from the coast. Why is it dangerous? This is because they're an aggressive species of shark that tend to hunt in areas where people swim. Should I be worried? Really, you should be more worried about this shark swimming on the beach than fishing in a freshwater river. Sharks must keep salt in their bodies to survive, and most can live only in salt water. But bull sharks have developed special adaptations—the way their kidneys function and special glands near their tails—that help them keep salt in their bodies even when they're in freshwater. Scientists are still studying these sharks to figure out why they developed this unusual ability. So, keep an eye out for these potential dangers.