[naiad]

Fascinating. The AAT sounds plausible enough. There are still some things that I don't understand about it though. These are:

Nostrils which cannot be closed. Most aquatic mammals close their nostrils when diving. No animal uses a noseclip or pinches its nose, and few use sinus flooding.

Poor underwater vision. Although some people are apparently better off than others, including tribes who dive regularly, most aquatic animals have much better underwater vision than we do. Those which do not, or live in murky water, have other adaptations such as long whiskers (seals), echolocation (dolphins and whales), lateral line (aquatic amphibians).