Sea Nymph was published in 1963 by Midwood Books. Peggy Swenson was one of the many pseudonyms used by the writer Richard E. Geis. Gies wrote 15 books as Swenson. According to the cover, Sea Nymph is about a woman who “wanted to learn skin diving but she learned a warmer sport.” Water polo, maybe?
The cover is by Paul Rader. This site has featured a lot of Rader’s work and it will probably feature even more before the year ends. As usual for paperback covers of the era, the cover hints at more than it shows. I assume that the hairy arm with the huge hand attached belong to her diving instructor. The violet tattoo is an interesting choice.