in order to swim and dive to that extent, they would have had to live near major water sources. I'd be curious which sources the book mentions. The Mediterranean? Major rivers?

found around the Mediterranean, in the eastern part and in the west on the Iberian peninsula. Inland regions where they lived might have included lakes or rivers now obscured, lost, or altered by glacial scraping and melting. There are Neanderthal and Denisovan remains near inland lakes in Asia.

I'd guess that the "swimming" involved more than recreation and was related to obtaining fish and seafood with nets or spears.

They have an episode on this. It's interesting.

They have tones of good topics, if you like podcasts, it's a good one to grab. It's smart and fun

In the article the author actually goes on to postulate that much of human (hominid) morphology can be explained as driven by a semi-aquatic existence. Thus extending his claim well beyond (and back of) Neanderthal. That would seem to be a stretch. But the bone formation in the ear (if really specific and indicative to the claim in Neanderthal) is really pretty interesting in and of itself. Water, when available, has always been a rich resource for nutrition. No reason to assume that an adaptive hominid line wouldn't exploit such an opportunity. Although labeling such species "semi-aquatic" .....

in her book, "The descent of Woman."

The Guardian
https://www.theguardian.com/science/the-h-word/2013/may/13/aquatic-ape-elaine-morgan-history-science

"It was while reading The Naked Ape (by Desmond Morris) that Morgan first came across the idea of an aquatic phase in human history. She contacted Morris and learned about the theory's architect – Sir Alister Hardy, then the Linacre Professor of Zoology at Oxford University. She wrote to Hardy for permission to develop a popular science book in which she expanded his ideas as an alternative to the savannah theory. Hardy had no objections."

Wikipedia

"Elaine Morgan OBE, FRSL, was a Welsh writer for television and the author of several books on evolutionary anthropology, especially the aquatic ape hypothesis which she advocated as a corrective to what she saw as theories which purveyed gendered stereotypes and thus failed to adequately take account of women's role in human evolution."

Famous Scientists
https://www.famousscientists.org/alister-hardy/

"Alister Hardy is best remembered for his controversial aquatic ape hypothesis, which says that our ancestors evolved by natural selection in a watery environment. Hardy believed his hypothesis explained a number of human traits – our lack of body hair, for example."