A fish can live underwater, and can also breathe water.

This is the opposite of a human, who can do neither, until drowned.

A shark is a bit like a fish, but bigger, and angrier.

I once saw a shark so angry, it ate a whole girl. Of course, I'm talking about a film.

Anyway, fish are really slippery, because they are always wet. And also because they are covered in cod liver oil, which is what comes out of a cod.

The most slippery fish is, by simple deduction, therefore the cod, since it is perpetually covered in its own filth!