Hairlessness lets us lose heat very rapidly, so we can run for very long periods, far longer than any other hunting mammal. Further, the pinnipeds (seals etc) are all covered in hair, and are conspicuously aquatic. Neoteny is common in higher mammals, particularly predators and primates.
In fact, most of the reasons he cites are actually good evidence for our hunting (carnivorous) past. There are much better pieces of evidence for our semi-aquatic ancestry.
For example, for proper neural development we need fatty acids found in large quantities only in fish: no other primate has this dependency.
If new-born babies are submerged in water, they automatically hold their breath, and reflexively tilt their heads up as they are brought to the surface.
Finally, though Elaine Morgan's views are not yet entirely orthodox, a substantial minority of biologists now accept them.
STEPHEN R GOULD