There's the Fastitocalon, a turtle the size of an island.
Good, bad or neutral?
@Thorondor It's just a normal creature. Aside from its size, there's nothing unnatural about it.
In the context of Tolkien's legendarium, the Fastitocalon might be only a mythical creature in the poem and not a real animal of Middle-earth. Tolkien didn't write about any other sea monsters (unless we do count the Watcher in the Water), but that doesn't mean that there weren't any others at all. If nothing else we can imagine some sea-dwelling mega-fauna that are analogous to creatures alive today: whales, giant squid, huge eels, giant sharks, etc.
I came up with one myself as a home-brew for The One Ring Roleplaying Game: the Brimthrowend.
The Brimthrowend (Old English: 'sea scorpion') are intelligent, highly organized creatures that are kin to giant sea scorpions that thrived in the time of the Spring of Arda. Their unique characteristics and form of communication developed during the long night of the Sleep of Yavanna. Brimthrowend came to inhabit the ruins of Nogrod and Belegost (and perhaps other, flooded Dwarf-cities and mines) as a result of the defeat of Morgoth and destruction of Thangorodrim at the end of the First Age. Their presence was one of the reasons that the Dwarves eventually gave up their attempts to restore their old cities and migrated east to Khazad-dûm. Colonies of Brimthrowend might also be found inhabiting networks of coastal sea-caves. A colony might number in the hundreds or thousands.