Keep a log that has the date, odometer reading, and how full the tank is. All you need to know is how many gallons the tank takes, how much you used, say habe a tank, and do the math. 16 gallon tank, use half a tank, 8 gallons X $5 you owe $40.
I think have some sort of agreement never to return the boat with less than an agreed upon amount of gas, especially if there is no gas sold where you keep the boat.
If for any reason you are worried that the other two you are buying the boat with will not be honorable in paying for gas, don’t buy the boat. No matter what it is very risky, especially if you are splitting the boat because you cannot afford one on your own. I know people who have owned boats, trailers, and condos together, sometimes it does work out ok, but for sure every time it worked out without arguments or a lot of trouble all parties actually had enough money to own the property outright themselves.