The shark is a mob that recently came into use for Shark Teeth.
When you collect 75 Shark Teeth and 
30000 you can eventually talk to Jin at Fishman Island to obtain Fishman Karate (Requires you to have the Fishman Race first).
Sharks have 70 health.
Sharks will appear when a player has been in the water for an extended period of time, and will deal multiple quick ticks of damage depending on your max health (%) every .2 seconds. The shark will instantly kill a player, without needing a grip animation. Which can be useful for doing the Kenbunshoku quest that requires you to Perfect Block 200 times as their rapid attacking allows for easy perfect blocks.