Why is it not acceptable to say that the mesosaurus could have traversed from one continent to another via the ocean?

It is not acceptable to say that the mesosaurus could have traversed from one continent to another via the ocean.  Although Mesosaurus is a reptile that could propel itself through water with its long hind legs and limber tail. There is no possibility that the Mesosaurus would have the ability to swim across such a large body of ocean because it was a coastal animal. 

Because some people pollute water ans waste it