Only one choice for me, Carcharodon carcharias more commonly known as the Great White Shark. The ultimate predator of the seas.
Anywhere up to 1,500 kg of eating power with lots and lots of teeth.
Also, you get to patrol anywhere between 1 and 1280 meteres of depth.
Edit - saying that, the extinct Carcharodon megalodon weighing in at 13 metres and up to 20 tons looks an even more interesting fish to have been (especially as its teeth grew to more than 6" long.) Some people postulate it fed on large baleen whales.