High metabolisms. Needed to breathe frequently, therefore needed to surface, therefore needed to transit the boiling hot surface to get to the air, and were unable to shelter in deeper water.
Tunnel dwellers are accustomed to lower oxygen and higher carbon dioxide in poorly ventilated burrows...
That sounds like a good explanation. Whether or not they were warm-blooded is an interesting question. I'm sure the therapods were, based on what I have read. Leatherback Turtles have been shown to sxhibit some degree of warm-bloodedness.