DrRog said..
What's the shower setup, Cisco?
The vanity sink in the head has an 8 litre capacity. It's drain has a T piece with a stopcock on each outlet and hose leads, one leading to overboard and the other to a foot diaphragm pump feeding the shower head.
For a hot shower turn off both stopcocks. Boil or heat two 2 litre kettles of water, fresh or sea water. Once heated pour into vanity sink and add another two kettles of cold water to suit your temperature preference.
Open the shower stopcock and and pump enough to wet yourself down. Stop pumping and soap yourself up. Once you have lathered and washed yourself, pump again to rinse off.
Once rinsed you should have 3 to 4 litres of water left for the pure luxury of hot showering. I use a shower head on a flex line so I can put water directly onto the hairy regions.
A good quality well insulated engine waste heat water heater can save time and gas used heating water. I have used one that produces really hot water and keep it stored for up to 48 hours.
I am not a fan of pressurized water systems on a yacht because if it develops a leak it will dump all your fresh water to bilge.