Difference of the sea of the underground world and our world?

The travelers build a raft and sail the underground ocean. They find several prehistoric creatures and are attacked by them. They also see afight between Plesiosaurus and an Icthyosaurus. Our sea does have very old life in it but they don't look like monsters from a Godzilla movie.

Nevertheless, if from the distance which separated us from this creature, a distance which must be estimated at not less than a dozen leagues, one could see the column of water spurting from the blow-hole of the great animal, his dimensions must be something preternatural. To fly is, therefore, the course to be suggested by ordinary prudence. But we have not come into that part of the world to be prudent. Such is my uncle's determination.