Small on demand hotwater heaters require a few seconds warm up time, EVERY time you turn them on. Then they only maintain a steady temperature if you leave the water on. They are NOT appropriate to saving water by getting wet, turning it off, soaping up, and then rinsing. I don't consider them a good cruising alternative.
, you could have a very small, (5 gal) standard home unit. Then, when cruising, use a solar
heated, 2.5 gallon black plastic garden sprayer with a thumb actuated dish washing
nozzel on the hose. We get two nice showers with ours, including washing
hair. It really limits water waste.
In moderate climates, we let it sit in the sun a few hours in the afternoon, and this provides shower
water at a perfect temperature around sundown. In cooler climates or cloudy days, we spray a tea kettle full and heat it on the stove until it almost whistles.
After poring it back in the sprayer, it is the perfect ratio of hot to cold to make a nice warm shower.
We shower in the cockpit
, and this keeps the residual moisture out from down below, where we don't want it.
It is simple, cheep, last decades, and a really nice shower...
BTW, We can collect rainwater in the dinghy
, jug it, and later use THAT water in the sprayer for showers, or otherwise washing clothes. This would be water that is OK for this, but you would not want in your tanks