Squid

Mega Shark vs Giant Squid
Jaws (duh)

(from listology)

1. The Abyss: Feel good holiday movie of the season! Woo! Characters that you want to die, do, in spectacular fashion, and everything works out okay for the sea monsters in the end. Only they're not really sea monsters. Kind of. Whatever. They're *purdy*. The movie's enjoyable in a family movies at the holidays kind of way, and did I mention that the sea monsters are all *purdy*? Maybe only in a way an oceanographer could love, but still. Six thumbs way up.
2. Crocodile: Technically more of a river monster movie, this is a big ole stinker from Tobe Hooper, who should know better. Worth watching for several bizarre scenes, including a physics-defying dog, reptile bulimia, and the bit where the crocodile goes all airborn a la the General Lee in Dukes of Hazzard.
3. Frankenfish: Finally! I give this movie 5 full fins up. Great setting, appealing lead character, voodoo swamp queen, swamp loner, and hot lesbian biologist. Rrrowr! But what really makes this movie is the fact that the fish shoot 6 feet into the air, and that the ending is upbeat, with hope for the fish to come out on top. HuhZAH!
4. Lord of the Rings: Fellowship of the Ring: Where a sea monster makes an all-too-brief but unexpected and welcome appearance.
5. Deepstar Six: What if you threw a sea monster movie and none showed up? The sea monster is MIA for an hour and 20 minutes, after which the movie is watchable. Also, I was unaware that there's some type of undersea regulation that all women serving with sea monsters wear white shirts at all times, just in case they fall in the water.
6. Peter Benchley's Beast: Watchable, but only once. Most of the suspense comes from not knowing whether the CGI-giant-squid or William Petersen will chew through the scenery first.
7. Peter Benchley's Creature: Surprisingly decent and cheezy by turns, it's equal parts renegade shark-thing and Navy conspiracy flick, all set to a funky island beat. Watchable.
8. Chupacabra: Dark Seas. Chupa-riffic, but really kind of a killer-goat-on-cruise-ship movie.
9. Orca: "There has never been a substantiated case of an orca killing a man, despite the 1977 movie Orca, in which a killer whale seeks revenge on Richard Harris by eating all his costars. The movie was so silly, unscientific and unbelievable that one critic suggested Harris fight a duel to the death with his agent for getting him the role." -- Tim Cahill, "Kayaking Among the Ice Children"
10. Godzilla vs the Sea Monster. Thanks AMC Monsterfest!
11. Deep Rising: Oh what's not to love? The snappy dialogue, the plucky boat music, and the unbridled gore. Awesome.
12. Ghost Ship: Less lip, more monsters. Totally qualifies.
13. Deep Blue Sea: LL Cool J as a religious cook versus super-intelligent sharks? Oscar gold.
14. Lake Placid: Sea water, fresh water, still a great big aquatic fiend. "I'm rooting for the crocodile. I hope he swallows your friends whole."
15. Piranha: Does anyone else remember this movie? It was wrong on so many levels. People, those bad fish ate a summer camp. That's hardcore.
16. Piranha Two: The Spawning: A movie so bad, I forgot I'd actually seen it until now.
17. Titanic: Is there anything scarier than Kate Winslet's hat? No. No, there is not.
18. Lost Voyage: The terror is unleashed: Judd Nelson returns to the screen.
19. The Jaws series. The first one was awesome, especially when remade in 30 seconds with bunnies. (Seen: Jaws, Jaws 2, Jaws: The Revenge)
20. Shark Attack (and 2, and 3 ("Megalodon!")): Aka The Movie That Ate Casper Van Diem's "Career". So bad, even for sea monster movies.
21. Haunted Sea: This sounds fantastic. Why doesn't Netflix carry it?
22. Alligator: "A gigantic alligator with a voracious appetite terrorizes the unsuspecting (and delicious!) population of Chicago in this tongue-in-cheek thriller directed by killer-animal specialist Lewis Teague (Cat's Eye, Cujo)."
23. Alligator 2: The Mutation: Which gets a separate listing simply for deciding to take itself seriously.
24. Shark Hunter: The SciFi channel are just good people.
25. Deep Shock: Did I miss the announcement that September is Sea Monster Month?
26. Dark Waters: "After an oil rig in the Gulf of Mexico is infiltrated by bloodthirsty, bioengineered sharks, a team of underwater specialists is called into action to investigate the situation and destroy the beasts. This biting thriller stars Lorenzo Lamas (TV's Renegade) and beautiful Simmone Jade MacKinnon".
27. 20,000 Leagues Under the Sea: I don't believe any comment is necessary.
28. Snakehead Terror: Boo yah! Fish! And snakes! In water!
29. Dagon: Oh SciFi Channel, how do I love thee? Let me count the ways. One, filming Lovecraft stories. Two, filming them with a maximum amount of cheese. Three, making them dark and mysterious, if not completely unscary.
30. The Beast from 20,000 Fathoms (via Odysseus)
31. It came from Beneath the Sea (via Odysseus)
32. Mysterious Island (via Odysseus)
33. The Deep (via its twin water-logged crushers. Heh.)
34. Spring Break Shark Attack (via JohnnyW)
35. Free Willy, Free Willy 2, Free Willy 3: Return of the Whale-i, etc etc: (via 1922). Ranked right up there with Where the Red Fern Grows for excessive weepiness. But still, in the interests of accuracy...
36. Leviathan