Figure a regular low-flow type shower head
puts out 3.5 gallons per minute. About half the flow with be hot water, the other half cold (less HW used if very hot from engine, maybe more if heated electrically and in a cold climate). In Mexico
use about 25% hot and 75% cold. Let's say 2 gallon per minute hot water use worse case, which would give 3 minutes shower run time. If wanting to shower like at a hotel
, not much, but if used to budgeting water usage on board this is a lot.
I would say 6 gallon tank. Good thing about it, if the hot water runs out whoever is in the shower will stop draining the flipping water tanks