preservation and monitoring are important to you then I thought I would give a couple of thoughts.
Install a cheap
meter so you can see the usage.
You dont have to look at it often. What is interesting is how much water is used for different functions. You can see how much water has been used for a shower
When guests know you have this, then they naturally will conserve more.
If you install another meter on the fill system, then you will get to see how much was added to the tank.
You can then calculate exactly, down to the liter, when the tank will run out. (If you really wanted to)
First time on a boat guests should always be given the job of filling one 20l container of water and lugging it back to the boat.
It gets the point across far better than any talk on conserving water.