That's an interesting question, and I guess only you can decide what is best. Some other things to consider before you try to take out a loan for a boat though: What kind of shape is your car/truck in? Will it need to be replaced before your income goes up much? Will you need money for college, rent, etc. Do you want to make payments on a boat for 10 years, it's a long time. What other debts do you currently have or expect to have? Always think of things like this with the mindset of "What will I have to give up?" It's called Opportunity Cost, basically if you do "x", then you can't do "y".
I'm 32 and I just bought my first new boat a month ago, I've owned several other boats that were between 15 and 20 years old and never had a problem with any of them. Keep in mind as well that a boat loan may not be easy to get when you're only 19 or 20 because you haven't established much credit yet. $4000 down will definitely help though. Also, how often are you going to use it? If the answer is every weekend, then it would be easier to justify a new boat.
If you know there a lot of good used boats in your area, that may be a great place to start. I think if it were me, I would go the used route for now, but only you can decide what is best for you. That's enough rambling from me, good luck with your new/used boat.