To have a sensible shower
in a temperate climate you need about 8kW plus., as well as a steady reliable water pressure and flow otherwise you get scalded. There are plenty of domestic units in the UK but most if not all are not rust proof.
I recently purchased the Eccotemp L5 as a temporary shower
while doing major house renovations powered by a propane
cylinder. It is absolutely brilliant but it is unvented so must be used in a well ventilated area otherwise you die from CO poisoning. It does need a fairly high and steady water pressure otherwise it shuts down, so the the average boat water pump probably won't cut it, plus again you get scalded if the pressure drops.
If you cannot provide the power either gas or electric
and a steady pressurised flow of water then an instantaneous water heater for a shower is probably not good enough, but ok for other uses. The advantage of a stored hot water tank is that the water temp remains pretty constant during pressure fluctuations as both hot and cold feeds are affected similarly