I think a 6 gal is fine for a couple. If you like to take a shower is is about cleaning, then I think it should get the job done for 4 people most of the time. Having said that, if you want to act like you were ashore, then 6 gallons wouldn't work. I can't imagine using the same amount of water as I do at home unless you're on a mega-sailboat (e.g., a Perini Navi -- but then, that wouldn't be a likely place I'd be showering ;^))) )

I had a friend who took his 38' boat to Bermuda with 5 people on board. His approach was to turn-off the pressure water before each person took a shower. Then they could do whatever they wanted ;^))).

You can play with the theromotat and increase the temp a bit -- but be careful, you can get scalded.

P.S. - The only caveat is that overnight, a bigger tank will give you more effective warm water for an early shower.