Hi EP

Some large herbivores could float quite well, pushing themselves with the occasional step to the bottom - weird trackways seem to be caused by such a scenario. But others were balanced all wrong and they'd actually capsize if their feet left the bottom. We do know the predatory dinos could swim too, so it was hardly an escape for any herbivore to hit the water.

Thermoregulation is actually harder in water because it conducts heat away so well, though some ultrasaurs were in danger of being too hot, even if "cold-blooded", because their massive bodies insulated too well. Perhaps they cooled off with a swim?