Most of the nudity was provided by Rebecca Louise in a flashback sequence.
There is a skinny-dipping scene in which Rebecca Louise is naked with another actress. The closed-captioning identifies this character as “Lisa,” and IMDb says that Lisa was played by Anouk Samuel. While all of that makes sense, I can’t say for certain that the second woman is Anouk because the woman does not seem to be the same one pictured on her IMDb page. That could just be a hair-color issue (the character is a blonde, Anouk is not), and the ID may be correct, but don’t accept it as gospel for now.
Our girl Sydney Sweeney was sexy, but not naked.