It's not entirely clear whether Marvin is one of a kind or just the last of his kind and he's been tight-lipped about it.  In fact, even if you figured out what Marvin was, you wouldn't learn much since there's little on Wikipedia and what little there is falls into that category of maybe fairy, maybe demon, possibly were-beaver or maybe just a crocodile eating a beaver.

Whatever he is (just a regular old guy is always a possibility), he came to Port Washington right after the Mayor's invitation and it's a given understanding that he knows the docks and the local waters better than just about anyone else in town.

What else is known?  Well, he's tight-lipped, which makes him an ideal accomplice (for a price), Welsh (he can't hide the accent), gruff (he can't hide the scowl), short (even shorter out of his boots) and passes as a credible pirate (he dressed as one last Halloween much to the delight of the local children).