I think you'll be fine buying a boat and being 17, consider yourself lucky and ahead of the curve.
Make sure to get the title for the boat when you make the purchase. Or at least a bill of sale and keep the seller's info! The last boat I bought was from a great guy in Canyonville and when I moved to CA I had a heck of a time getting it registered here. The trailer didn't have a # or license which are needed in CA. Talked to the seller and got some forms filled out and no problem.
Best,
By the way, what kind of boat??