Sharks eat each other quite often. Sharks can also be eaten by any larger fish - if the shark is young or quite small, a barracuda or grouper could eat it.
sharks
Sea lampreys are eaten by larger predators like sharks. However, the most common predator of these creatures are human beings.
Well sea creatures eat bigger or smaller creatures!
They feed on fish and other small sea creatures.
The hippo and prehistoric creatures
I heard that salmon eats plankton, which eats microscopic organisms, but I don't know anything further than that.
No because it eats plankton and microscopic sea creatures.
Parrot fish do and other sea creatures
Lets see, fish, calamari, shrimp, lots of stuff.
Yes, but all were eaten by fish and other sea creatures.
Some sea creatures have ansetors of dinosaurs
Birds such as sea gulls, small animals such as opossums, raccoons, and other small creatures.
If you mean, "are all under water creatures, sea creatures?" then no. The term SEA is for the ocean and there also can be creatures in freshwater. Also some creatures that live in water such as some snakes and sea turtles come out of the water every so often, so they are no total sea/water creatures. Fish are underwater creatures though.