[eesmith]

"They would be not as good at four legged things as "normal" four legged animals, and not as good at arm things as "normal" armed animals."

So, like seals, penguins, walruses, mudskippers, and many others? That is, there are many examples of species which don't fit well as sea creatures or terrestrial creatures. But they are just as successful, evolutionary speaking, as you or I.

In any case, the evolutionary argument is our early tetrapodal fish ancestor found that it was (likely marginally) better to sometimes be on land - like a mudskipper. Since there were no other animals in that niche, the descendants who could be more effective on dry land were even more successful in having children, etc.

How well would you do if you had to live in a place which was flooded over your head for a few hours day?

It is really hard to think about "deep time" like this - time long enough for mountains to form and disappear is also time long enough for species to change by quite a bit.