Here at Boat Bits Central we have had a few emails regarding propane hot water heaters as opposed to going the solar shower route...
Fact is, most of the places folks tend to cruise even a solar shower is something akin to overkill and while we are in the dead of winter here in the Caribbean (according to my depth sounder the water temperature is 82 degrees), simply leaving your pressure sprayer/shower outside is enough needful warming for most folks.
So it goes...
I guess, when all is said and done, it's simply what works for you and your budget...