Their tails would certainly have been useful for driving them through the waters. When we look at the dinosaurs, we find them occurring well up in the geological column, laying eggs, seeing them hatched and also walking around on highest grounds without any sense of panic. This suggests that the Flood had subsided long ago and the dinosaurs were now living naturally in these areas covered by Mesozoic waters. We see that there is a gradual change in the types of dinosaurs occupying the world during these periods of time. For example, we have the appearance of the footprints of bipedal dinosaurs before those of the quadrupeds.
Dinosaurs appear before mammals, for example, in the rocks.
It is now well-known that dinosaurs reproduced rapidly. Their nests had many eggs. So, the recovery and dispersion of the dinosaur population occurred fairly rapidly after the Flood. Since the days of their dominance on the earth, they have disappeared, as have many other kinds of animal since. We do not know for sure what these causes have been, though there has been much speculation.