Sea Monsters? I don't think they'd look quite like that but they're definetely out there. Well, when I say that I don't mean maliscious lobster men or CRAB PEOPLE, but there are some scary things lurking in the deep. We've explored so little of the sea itself, new species are being discovered every day. And we all know about the bloop...
http://en.wikipedia.org/wiki/Bloop
Sure, we know about Giant/collosal squid, and of course there are whales that could survive down there. Just because we haven't seen it, doesn't mean there isn't anything larger.