What creatures lived next to dinosaur that could breath under water?

Dinosaurs were all reptiles and therefore, land dwellers. None of them could breathe underwater, although marine creatures that lived along side the dinosaurs have similarities to the dinosaurs, they weren't actually classed as dinosaurs. Therefore, no dinosaurs could breathe under the water :)

EDIT

Fish and some amphibians are the only vertebrates that have gills and can breath underwater. The same applies to ancient animals too.