I found a February 2010 article in Nature that gave detail on a dinosaur that may have lived similarly to an amphibian or even a fish, yet it did not display typical anatomical characteristics for living in water such as propelling tails, flexible fins, or webbing. It was known as a spinosaur (Baryonyx walkeri), and its head resembled a crocodile with unique cone-shaped teeth. Most people probably think of a dinosaur as a land-inhabiting creature, but research suggests that this specific group was not restricted to land.
One important find revealed that the long snout and tooth shape were evident of fish predation in spinosaurs. T. rex had serrated teeth that were perfect for its carnivorous diet, but B. walkeri displayed piercing teeth prime for fish feeding. This behavior was determined because of partially digested fish scales that were noticed in a 1983 fossil from England. Unfortunately, pterosaur (non-aquatic dinosaur) remains were also found in the fossilized spinosaur gut. The presence of oxygen isotope concentrations in turtle shells, and in the tooth enamel of spinosaurs, other dinosaurs, and crocodiles at this time period were looked into for further claim at whether the spinosaur was aquatic or not.
The reasoning was that the concentration of oxygen-18 in spinosaur tissues should match that of crocodiles and turtles if spinosaurs could be considered aquatic. There were obvious differences in concentrations between spinosaurs and terrestrial dinosaurs, but similar concentrations between spinosaurs and the aquatic animals used in this study. One argument against this states that a diet consisting of mainly fish would lower 0xygen-18 concentration levels to that of aquatic animals such as turtles and crocs. It is a controversial issue, but but may prove accurate with further studies. All in all, the central theme I found in this article is that it may be difficult to distinguish spinosaurs as aquatic dinosaurs since the limbs of their skeletons are completely atypical of common aquatic limbs utilized in propulsion and swimming.
Reference: doi: 10.1038/news.2010.84