Here's a list of all the monsters, starting with the playable ones:
Talasamodon: Swims slow and does nothing.
Hendus: Only useful for getting in the shallows portal. It sinks when you stop moving and gets exausted quickly.
Ichtyosaurus: Can swim pretty fast and dive deeper than other monsters. It is also pretty powerful and can be used to reach the cave portal.
Tylosaurus: Armed with a powerful bite and the ability to smash loose rocks to pieces, this monster is used to reach the volcano portal.
Dolichorinchops: A small pliosaur that swims fast and can jump out of the water. It can't go very deep but can help you reach the rock portal. It can also swim into the shallows portal without any troubles.
Nothosaurus: The final monster, the Nothosaurus can climb on beaches and dig up fossils. This monster can also fly. (See the cheat "Flying Lizard")
Next here are the unplayable creatures.
Archelon: A turtle that is about ten times bigger than Hendus and swims near the shallow area.
Kronosaurus: A pliosaur like Dolichorinchops but much bigger. They usually hang out wherever you see Tylosaurus. There's also one swimming near the shallow area.
Xiphactinus: These are big fish with large jaws that swim in groups of three. A bunch of them are near the open ocean portal.
Jellyfish: A clear jellyfish that does nothing and for some reason cannot be eaten.
There's also a kind of bird that has big feet that I forgot the name of, but there's a pack of them that appears near the dolichorinchops pack and can be eaten by tylosaurus.
Cretoxyrina: A shark. One appears near the rock portal and others appear in the Rock Zone and the Shallow Zone.
Fish: Just...Fish. They can be eaten by any monster.
Ammonite: Squid with the ram-horn shells that can be eaten by Tylosaurus.
Squid: These glow-in-the-dark squid are most commonly seen in the depths of the sea and are eaten by Icthyosaurus.
Giant Squid: A very large squid that only appears in the Open Ocean.
Shellfish: The game booklet said that these can be eaten by hendus, but I'm not so sure.
Well, that's the end of my list. Hope this helps you with surviving in the ocean!
You can get Hendesus's head by going to the first challenge portal and selecting the challenge that says it will result in the scull of a new creature. P.S. challenges are purple on your radar and hendes is the turtle. You have to get termondontosoures in order to go really deep. To get more fossils you need to take the plesiosaur to the place were there is a fossil down deep then go next to it were there is a not so deep place if you play your cards right you come out the other side with reduced health that you can eat back with fish.