What advice would a Grammar-Nazi editor give about that one?
* Try to keep the same subject in successive sentences when it's practical. Change 'The rest of her remains ...' to 'Otherwise, she remains ...'
* I suggest the preposition 'of' after 'the skin', instead of 'across', is slightly better. Note how that changes 'the skin' into the genitive case of a collective noun, thus 'are' must then be changed in the singular 'is' too.
* Even better, restructure the sentence so its verb immediately follows its suggest. Perhaps, 'Her skin has different shades of tan on the arms and shoulders, like water stains in a bathtub.' Note how much more personal that would feel to the readers.
* Also 'so' is the wrong conjunction to use in final sentence. 'So' is used when a dependent clause is a consequence of the main clause of the sentence. But here, your main character had two objects competing for his attention but he did not select either of them. You need to change that 'so' to 'but'.
Overall, the writing is of high quality, so why not give readers more, eh? Add more decoration to it. Personally, unless the female character shaves, I would go for another metaphor likening her pubic hair to fungus. Can you see how that subtly enhances the dosshouse-bathroom image you have already established?
Keep up the good work. Send your cheque to ...