My personal preference is to always shower on the boat. I am less than impressed with the shower 'offerings' on shore, and never know what I am stepping into.
I was taught on my initial trip in 2000, by a great skipper (now my husband), that the squirt, stop, lather, stop, rinse, towel method can be very refreshing and does not take that much water. I think most guests will comply when they understand water is in short supply.
I would much rather fill the tanks at VG mid trip and continue on. We are sailing, snorkeling and swimming all day.....I just want the salt off.
Others may disagree, but this is my $0.02 worth.