Keep track of how many miles each puts on the boat. Figure out how many miles per gallon the boat uses and divide by usage. I know you didn’t ask my opinion on this, but it is a bad idea to buy a boat with friends. If one loses his job, if his wife S/O doesn’t want the money going towards the boat, etc., then only two will be stuck with all the payments and insurance and repairs. You should get all this hashed out before you buy the boat. I know this first hand as I had a S/O that bought a boat with his best friend of 20 years and the friend lost his job, my S/O was stuck with payments he couldn’t afford.