On the Island Princess and Pacific Princess, two lovely old vessels, the showers were so small, just about every comedian had a joke that went like this.
" I finally discovered how to take a shower. You soap the walls and then rub your body parts against the walls. And never try to bend down to try top pick up the soap."
To be practical there are ships ply and I recollect some late versions of Princess sh ips plying the oceans that have larger stall showers, especially the newer Carnival ships.