Although there are still many controversies regarding the consistency of dinosaur like rather if it's feather or not, or what is the lifestyle and habitat of them, most dinosaur that are believed to be land animal are indeed land animal.
This can be verified with a matching footprint fossils, the location of the fossils, and the depth and the component of fossilized rock they are being excavated.
So, saying a dinosaurs is a fish is rather incorrect unless what you meant is their ancestor since every complex animal in land evolve from the sea.