Central American rivers, like rivers in the United States, flush out debris from far inland. Curious items drift by en route to the sea--whole trees, banana bunches, the occasional dead cow. The river outlets where this motley flotsam spews into the ocean contain a rich stew of nutrients that attract small and large fish, including hammerhead and bull sharks. In looks alone, their eyes set on T-stalks protruding from their heads, hammerhead sharks are grotesque, scary creatures. But a unique trait makes bull sharks potentially more dangerous: These saltwater creatures can survive in fresh water. Stocky and broad-snouted, with high-sailing dorsals, bulls freely enter freshwater rivers. In Central America they have been known to swim more than 100 miles up the San Juan River to Lake Nicaragua, where they've killed unsuspecting bathers wading near shore. If you hook a tarpon at the mouth of one of these rivers, there's a good chance you'll lose it to a shark.