Came across a few books that might be of interest. I am assuming these are now public domain but have no direct knowledge one way or the other. Last two published by the Navy have some nice photos. Can be downloaded as PDF files.
UNDERWATER EXPLORATION - LADYBIRD BOOK
by LADYBIRD SERIES
Topics LADYBIRD SERIES, BOOKS FOR CHILDREN, ENGLISH
https://archive.org/details/UnderwaterE ... q=cousteau
The World Beneath The Waves
by Dr. Gilbert Doukan; Published 1957
Handbook of Naval combat underwater demolition team training
by United States. Bureau of Naval Personnel. Training. Standards and Curriculum Division; Published 1944
https://archive.org/details/handbookofn ... er%2C+1942
Naval harbor defense operational manual
by United States. Office of the Chief of Naval Operations; Published 1946
Topics United States. Navy, Coast defenses, Coast defenses
Published in 1946 so it even covers German efforts. Section VIII chapter on "Sneak Craft" describes mini-submarines, and some mention of WWII divers including Italy, Germany. There are a few mentions to Japanese swimmers. Word search Swim to find references to other sections covering diving.
“A skin diver is a fellow who pulls on a pair of fancy swimming trunks, some rubber fins, a diving mask and canvas gloves, then fills his lungs with air and noses down into the ocean looking for two fisted trouble.”