I have some ideas on how party boat operators should set up these facilities. First, one of the big problems with boats running out of water on long trips is because they use traditional heads. One guy urinates and then uses 2 1/2 gallons of water to flush it down. Then so on and so on.
Instead, they need to incorporate a urinal system like you find in port-a-potties. You urinate in a trough and do your other business in the traditional head. The trough would drain directly into the holding tank. This would eliminate the boats running out of water on long trips. It would also keep the traditional head cleaner for when you did have to do the other business ;-)
Here is a pic of what I'm talking about.
Now, for showers, I'm not a fan of having to shower right next to a toilet. I think you should have one restroom for showers and one restroom for business. The problem is, and it seems to happen to me on each trip I go on. One woman ends up going on the trip. That messes up everything. She ends up getting a personal restroom and all the guys on the trip have to share the other one. I have no answers on how to solve this problem.
LJ93