Center Console Boats And More

How can my family split the gasoline costs for our new boat?

Oh boy. Trouble brewing already. I just purchased a new pontoon boat to be used by my son, my son-law, me (and other family). The marina's gas dock closes at 5pm daily, so there's no way we can have a "always fill it up when you finish" rule. No one likes the "use it or lose" option as it'll always be empty. My only solution is, since the marina will put all gas charges "on my tab" monthy is... each time someone takes the boat out, they write down what the engine hours are prior to going out and/or when finished. That way, I'll split the monthly gas costs based on engine hours used. Not perfect I know; but, how else can a family do it when it costs $20 to just drive across the dang lake??? (by law, we can't carry gas our own gas into the marina).
LOL... I'm already chuckling about these answers. Did I mention my son and I are anal about this type of stuff?? (not to mention "cheap"?)