All animals and plants change gradually, so anything not yet discovered in the ocean would be different from what it was like when dinosaurs were still alive. Did you know birds and dinosaurs are relatives?

Also, we haven't discovered everything yet; but we're mostly discovering tiny animals like insects, not large ones.

How long do sea creatures live? Whales are the longest lived [and largest] mammals - they can live over 200 yrs. The longest lived of all animals is a type of clam in the North Atlantic ocean that can live 500 yrs!

1. Here's a good description of why people think they find something scary in the ocean:

The open ocean can be a terrifying place. Miles from shore on storm-tossed seas, with nothing but water in all directions--including straight down--a sailor or fisherman cannot help but wonder what lurks in the depths. When the oceans were still unexplored, these fears often took the form of imaginary monsters.

2. The whole article is really useful, and it's from a museum that knows a lot about science: Sea Monsters - American Museum of Natural History sea monsters

Many sea monsters include features from living animals. A large tentacle becomes part of a monstrous sea serpent or many-armed kraken: The eye sees a ...

3. Some animals that live deep in the ocean are larger than the ones not so deep in the ocean, but those are all animals without backbones; so they aren't dinosaurs. Here's an interesting article from Wikipedia:

Deep-sea gigantism - Wikipedia read here