"Deep Star Six" comes to mind. Ken

I don't know if you call them a monster, but one of my all-time favorite movies is The Abyss. And the DVD release with deleted scenes puts a whole new spin on the theatre release.

There's a lot of water monster movies out there. Tops among them are two 50s gems: "The Creature from the Black Lagoon" (most of it was underwater; you don't need an ocean for a water monster!) and Ray Harryhausen's "It Came from Beneath the Sea" (the giant octopus, only they built it with only six tentacles, because of budget restrictions. No matter, it still wrecked a lot of S.F.!) Not to mention "The Beast from 20,000 Fathoms", the giant Nautilus from "Mysterious Island", the giant squid from "20,000 Leagues Under the Sea", Godzilla, Gorgo and every B-movie monster on the late show. (Hmm... does Moby Dick count?)

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The Beast From 20.000 Fathoms is an all time favorite of mine. The mother of all "atomic bomb releases giant angry reptile" pictures. The whole godzilla franchise was modelled after this, and almost 60 years after it's release the genre is still going strong in films such as Cloverfield. I will never forget the image of the Beast shaking that cop like a terrier does a rat! One of Harryhausen's finest!

http://www.youtube.com/watch?v=CNCRtoFA8sc

I didn't think of Cloverfield when this came up, I guess because all of the action takes place on land. Cloverfield is one of the very few films I have watched where I actually let out a scream - and I was watching the DVD at home! I loved this movie.

My all time favorite "Water Monster" movie would have to be Deep Blue Sea. The thought of super smart sharks out there is very terrifying. And of course we can't leave out Jaws.