There is no end to the variety of systems that you could use but at the end of the day it all comes down to "how do you use your boat" and how much
money do you have to spend". If you have an endless supply of
money and
electrical power whilst at sea then you can run water makers,
washing machines, water heaters and have long hot showers 3 times a day. If not then you need a system to suit your
electrical capacity. Heat exchange units from your engine to a
holding tank work well if you spend a lot of time at sea but then you must consider the difficulty of monitoring your water
consumption. Personally I use an
electric instantaneous water heater which is about half the size of a house brick and fits neatly beneath my
galley sink. It draws about 13 amps when I am in the marina and gives me all of the hot water I need for
washing and showers. Whilst at sea I use the good old
solar shower because it is free and it helps
monitor my water usage. My advise is do NOT use any type of gas water heater in you boat unless you want you life and your boat
insurance to be nul and void.