Albacore tuna is my default answer, but there are so many ridiculously cool things in the ocean, picking just one seems so restrictive.
I personally would like to vote for all cnidarians. Jelly fish are just neat, coral is such a cool contraption and if you really understand anemones.....damn! The colors, the symbionts, the middle ages style war weapons, the chemical weapons, and the gracefull way the bounce on the waves.... They just have it all.... and all with out a nervous system!! Yup I vote for all cnidarians.... the big ones, the small ones, the pretty, ones and the deadly poisonous ones. I think they are all cool.