I am going to be polite as possible in responding, but please forgive me. I don't mean any offense, just want to clarify.
I wasn't asking opinions on whether or not I should. I was pretty specific in what I wanted addressed.
If you must know, I am doing a complete re-design of my boats interior and am removing an entire side bunk completely. Nothing structural, just the bunk. When I do that, it opens up the possibilities immensely. In terms of square footage gained, I have gained more than enough to have equal to; if not greator than, your typical head w/ shower below decks. True, you will not be able to stand up, but you will be able to sit quite comfy and shower down.
I get that people love being better than others and love to dispense "wise" wisdom, but all too often that just becomes a preachy "I am smarter than you, therefore only do this."
I know specifically what I want and am developing a plan to get to that.
Tankless, propane run, water heaters remove much of the concern about space. Ventilated well, propane is very safe on a boat and this is exactly what I intend to do. If you haven't seen them, you should check them out. Small and designed for RV's and small boats; they are great. I have a buddy that has one on his boat and it is amazing. While its output is smaller than a large one, it is more than capable of getting water hot.
Second, I don't want a porta potty. I don't. Again, in removing an entire side bunk; I gain more than enough of a foot print to install a manual head that is of the smaller size.
Let me say this. I have a medical condition that necessitates the desire for a hot shower. It isn't anyone's concern, but I have very specific desires. I have an advanced degree and am more than capable of parsing out what is practical and what is not. I cannot stand "well, it hasn't been done that way so it shouldn't". That isn't an argument, that is a statement of inflexibility rooted in dogma.
Finally. I apologize. I don't mean to offend anyone, but I felt the right to respond since I created the thread with very specific questions. Everyone has the right to their opinion and I am expressing mine. I want to thank the few that actually responded to my question with actual helpful responses. Thank you.