I'm the oldest of nine kids. Until I was about 12, the largest hot water heater we had at our home was about 30 gallons. If you used too much (and my father of mother didn't have a problem), then you were in for a very "rough" time. (But then who got a shower before you wen to school, in those days? Maybe before bed...)

On our boat, we have wands to use to shower. If you have a shower stall with a shower head, it's an invitation to imagine that you're not a on a boat. Otherwise, I think it's pretty managable.

Larry's comment is correct about 6 gallons working -- as long as you didn't sail most of the day and haven't turned-on your generator for 20 minutes or so.

MOST of the water heaters have the same electric heating element. So if you're heating up the hot water heater with a generator or at a dock, then you only get so many watts out of the heating experience/hour.

Now if you shower 4, eat dinner, and don't want to use warmish water to clean the dishes, then you'll need to run the generator for 20 or 30 minutes. That's were a 12 gallon heater will make a big difference. If you have 6 adults that will showering, then I'd think about the 12 gallon unit. (Even then, we used to do it with a bunch of people aboard. I think it's probably doable with 6 gallons.)

The ISOTHERM units have much better insulation than the old Raritan ever had.