The biggest ongoing expense will be slip and storage fees. Slip fees can range from reasonable to bonkers. Trailering the boat down the ramp is an option, but then a tow vehicle and storage for when it's not in use are required. Having a slip is convenient for using the boat, but costly.
Also, getting fuel at a given location can be a hassle. The nearest gas dock might be a few miles away, or a lake you really like doesn't have any gas docks. The gas dock is convenient, but you do pay handsomely for the privilege. Anywhere from 40 cents to a dollar over buying gas at the gas station. And, fueling a boat from 5-gallon cans is often awkward and precarious. This can be skipped by trailering the boat to the gas station on the way to the lake.
Lastly, don't forget to add fuel stabilizer to the last tank of the season to minimize fuel issues next spring.