A hot shower!
I had a 10 gallon heat exchanger water heater on my last boat. The 13 hp engine had to run a very long time to make the water in the tank reasonably hot.
We usually used a sun shower...If there was not enough sun to heat the water we would heat it on the stove and pour it into the sun shower. Very hot water from the stove mixed with cold water in the sun shower made for a warm shower.
On a sunny day the sun shower got to hot for me. So, my wife would use it first and then I would add some cold water and take my shower.