Powerboats tend to have terrible fuel consumption rates. My little 21' boat is lucky to get 5mpg cruising. Bigger twin engine boats often see less than 1 mpg. Unless you've got a sailboat, you're probably limited to less than 200 miles of total travel on a tank. With some fuel cans you might be able to stretch that a bit, but for the most part long distance travel is out without a reliable method of resupply.
This. Especially if the engines are older and have carbs the fuel mileage is often measured in gallons per mile, not the other way around. Now, you're thinking you don't need to go fast, so you can run the engine really slow, or just run one engine if the boat has multiples. On a displacement hull boat that may work, on a planing hull you're going to burn just as much fuel per mile tooling around at idle.
The reason you have to look to big boats that most likely have multiple engines is because small boats don't have very high weight capacities. If you have a couple people, some extra fuel and water, and any kids of supplies that have any weight to them (like ammo or canned goods) you're going to overload a 20 or 22 foot boat pretty quickly.
Lastly, boats typically require more maintenance than modern cars. Your car will run 50 or 60k miles with just oil changes. Boats will require scores of hours of maintenance to achieve that level of use. I used to own a 1972 24' twin engine boat, and it's no exaggeration to say I spent more time working on it than using it. Having an engine that needs work while you're offshore could be a life threatening problem.
So, it's not a bad idea, you just need to know what you're getting into before you work it into your plans.
_________________Angry Peasants, musings on guns and the state of our world
Don't go around saying the world owes you a living. The world owes you nothing. It was here first.
- Mark Twain