You are right to conserve if you have limited capacity while anchored out or on a cruise. I have been out in the Gulf and on the Rivers for a week at a time and 60 gallons would not have worked. 4 people, who wish comfort even with conservation would need more. One boat I have carries 200 gal.water 350 gallons fuel, the other 500 water 900 fuel. My last boat, that I really cruised with carried a 1000 of both. I do not hook up, as many a boat has gone down because of it. I have type I MSDS so sewage is not a problem. For those on no discharge inland lakes they would have to pump out. The point here is a larger tank cannot hurt, but a smaller one empty may. If you have the room always get the larger tank. Resale would be affected also. most sailboaters are not out for the comfort aspect of boating where as Houseboaters are. I like things to work without playing camping. It all depends on your life_style_, entertainment habits and so on. I know there are many people on this forum with Houseboats that have everything home does without having to conserve anything. That is why they have a Houseboat.I like both Cruisers and Houseboats. I like to travel so I am going to have an adequate supply of whatever is needed. Why turn the wife off saying you cannot take a shower or in many instances fill the tub. Each person is different as are their requirements. What are you going to use the boat for is the key.