It all depends, sometimes you’ll find a bunch of them, sometimes you won’t find any. They spawn very frequently near large masses of shallow water— but the water must connect to the ocean. I find them in the mouth of the rivers near shallow water. More often then not they spawn in the south or near the red obelisk if you pace yourself towards the snow biome you will definitely encounter a few along the beach.
When you tame these guys far from home, keep in mind that the Megalosaurus CANNOT pick them up in its jaws when tamed. This caused me a long hike home. Keep it in the basket until you’d like to release it. More testing required to see if any mounts can carry it when tamed!