Another way to handle it, is to have a pre-sail conference with the invited guests and explain to them the need for water
conservation etc... and the living aboard
courtesies. You are not being a bad person, just explaining the operational limitations of the vessel. Make a game
of the water conservation, encourage everyone to see how little they can use. Help them get into the seafaring mode. Most people that come to sail on a vessel are excited to help out and do what they can, they just need the information. I wouldn't worry about offending anyone.