Were there any marine/aquatic dinosaurs?
There are a couple of specimens of a dinosaur species named Liaoningosaurus paradoxus that have been found with fish in their stomachs and skeletal features suggesting it was at least semi-aquatic.  It is a member of the ankylosaurian dinosaurs from the early Cretaceous period of China.  

From Fish Hunting Ankylosaurs( Dinosauria,Ornithischia) from the Cretaceous of China:


  With elongate and fork-like denticles of cheek tooth crowns,L.
  paradoxus has a dentition capable of penetrating into animals like
  small fishes. The carnivorous adaptation of the dinosaur is also
  supported by the ungual modification to a sharp claw in both the
  fore-and hind-limbs. The evolution of a shield-like ventral armor
  plate and the loose sacrum-pelvic connection suggest that L. paradoxus
  may have adopted an aquatic way of life,using the ventral armor plate
  to protect the body from underwater attacks; as such,the open suture
  between the neural arch and centrum of the vertebrae cannot be used to
  indicate the juvenile nature of the type specimen. L. paradoxus is the
  first carnivorous ornithischian dinosaur since dinosaur was first
  known in the 18th century and represents not only the first aquatic or
  semi aquatic example of armored dinosaurs but also the smallest
  species of ornithischian dinosaur so far known.




Image Source: [Paleontology • 2016] Liaoningosaurus paradoxus • Fish Hunting Ankylosaurs (Dinosauria, Ornithischia) from the Cretaceous of China