There used to be HUGE sea creatures in the geologic past, and there are still some pretty big whales out there right now. The size of creatures depends upon a lot of things, but the available food supply and amount of oxygen in the water seem to be most important. Still, these two things have to exist for a very long time so the creatures can evolve and overcome other issues, like predators. Monsters though? I doubt it. The current trend in available food and oxygen is downward--that's our fault. Besides, it depends on what you mean by "monster". So, even though we know less about the sea floor than the surface of the moon, from what we DO know about marine biology, "giant sea monsters" are just not likely---and will be even less likely in the future.
I teach this stuff