In the Star Trek: Lower Decks episode "Kayshon, His Eyes Open" (season 2 ep. 2) Beckett Mariner & Friends (except for Boimler) go to a large room full of sonic showers where people are cleaning themselves in full view of each other. Mariner explains to a crewmate (who is occupying Brad Boimler's usual spot) why Boimler is not present, saying something about his fear of "communal nudity." I could understand Boimler's shyness, and I doubt that he would be the only one unwilling to participate in this "public showering." It is likely that on a Starfleet ship with many different races, certain crewmembers would act like Boimler due to their cultural values and beliefs.
There should really be no reason why there are no dividers/stalls or even curtains to separate showers. In previous Star Trek shows, we've seen private showers before. Sure, they were usually in high-ranking officers' quarters such as Captain Archer or Lieutenant Torres, but they wouldn't really reduce the junior-grade officers to situations of such humiliation...would they? Are there private showers anywhere on the Cerritos? Or has it always been this way on all ships?