We've had various trips on both hire and private boats over several years.
A wooden broom or mop handle makes a good dipstick for the fuel tank...on our three week cruise from the Midlands to the Thames and back a few years ago, we found it invaluable. As we had to pay for our own fuel past the first two weeks, we wanted to make sure that we didn't run out or even worse, overfill the tank and waste money! Mark the stick with each dip and you'll know how much you're using and when it's time to fill up.
We've never had a boat with a water gauge, but we've never run out. We usually fill up every other day at least, when it's convenient. I'm sure we could leave it longer, but we like our hot showers!
Janet