Kevin
I have been amazed in the eastern Caribbean how many of the cruisers staying in a marina will use the marina facilities rather than shower on their own boat.
It is probably 3 out of 4 who use the marina.
(19 of 20 boats are sailboats)
I think you hit the magic word. I think sailboaters and former sailboaters who now have trawler type boats. Of those who have only been powerboaters, I think they're far more likely to use the shower on the boat.