There are more then 350 kinds of sharks. But only 30 are known to attack humans. A Bull shark can live in both salt and fresh water.
Sharks can smell blood from a mile away.
This attack usually happens right off shore. Also this attack happens from mistaken identity like when a guy on a surfboard looks a lot like a seal which is the favorite food of great whites and tiger sharks. In this attack the person hardly ever sees the shark because it leaves right after it bites. This attack usually doesn't happen. The victims for this attack are usually divers and swimmers.