Is an expensive speaker worth it just for the shower?

For most shower-only use, no. The consensus across verified-purchase reviewers favors compact, sub-$60 models like the JBL Clip 5, Soundcore Select 4 Go, and JBL Go 4, which deliver loud, water-safe sound in a form factor built to hang or perch. Premium models like the Sony ULT Field 5 are better justified if you also want a loud outdoor or party speaker.