I've had a look at some new FG yachts and the heads appeal to me since
they appear to be vitrually a plastic bubble with everything molded in
(the entire head becomes the shower, the sink in molded in, etc.) I
was wondering how I could mimic this concept in a steel boat. I could
possibly build-up layers of spray foam around formwork and plumbing
points (i.e. molded corners), sand it back, and coat with some
appropriate finish, possibly light matting and a gel coat.

Another possiblilty would be to find source of these little rooms from
a home caravan manufacturer or yacht builder and fit it into the boat.

Cheers, jimthom