Ok I just wrote like 5 pages then deleted it, since what you were really asking is: "I want to live on a boat when I am 18, how do I make that happen?"
I was a marine mechanic for 5 years and I can tell you that is the best job for a person living on a sailboat, you can basically live at your work so you don't spend money getting to work, and usually as an employee at the marina you will get a cheaper slip rental with good opportunities to buy a slip.
Something to understand about the marine industry is that you don't have to go to a school ahead of time. Once a dealer has paid their yearly dealer fees, they can send their techs to school for free at the manufacturer training sites, along with a lot of online and at dealer training. The hardest part is getting your foot in the door to get a job at a marina.
You will want to move, unfortunately you will have to be in an apartment for the first part of this plan. You need to be living near the areas where the training centers are located, ideally Mercury marine since they do I/O's and inboards and those huge pod drives now, along with the traditional outboards. As I recall they are all in Georgia, Keenesaw is Yamaha, I forget where mercury put theirs. There are other training sites like the ones in eastern WI but those only do limited stuff.
The reason for moving near the training site to find a dealer to work at is so you can drive to the classes every day from home, that means that the dealer who has already paid their yearly dues, can send you for free, even if you are just an entry level boat washer or something. If they had to pay airfare or hotel or basically anything, they probably wouldn't bother.
The classes start very basic (fundamentals of 2 stroke and 4 stroke engines, electrical theory) so you don't need to really know anything ahead of time, but it really helps to have basic mechanical experience.
Once you have been to enough classes you can move into a mechanics position. I have found that only by being a marine mechanic can you learn to be a marine mechanic, it takes years of experience. Once you have maybe 5 years of experience and lots of certifications, you will be able to move around to the marina of your choice.
As far as a boat goes, move up to a 22 footer that is trailerable next, they can be had for pocket change or free if you are very patient. Fixing a worn out but complete boat isn't expensive, its very cheap, its just super labor intensive and most people don't have skills.
Don't bother going any bigger and just save up until you are ready to move onto a boat (good mechanic job and live aboard marina) then get something 30+ feet that is nicely equipped and has a design you can live with.
DO NOT LIVE WITH ANYONE ON A BOAT. That's my best advice unless you have lived with someone in a one bedroom apartment (or better yet a studio apartment) for years and know it works, plus it would have to be one of those rare females that likes the crude sailboat life since you cant live in a space that small with someone you're not sleeping with.
So to summarize how to make your dream work:
1. Beg your way into any job at a marina that is a Mercury repair center that pays enough for you to get an apartment with a roommate or something. Must be near the Mercury Marine training facility so you can convince them to send you to the part of week or week long classes for free (probably with no pay). Negotiate that point before you are hired and tell them you want to eventually be a mechanic. Ideally you want the marina to be a place on the water with a ramp and parking so you can trailer sail for free, but if a land locked repair center is all you can get it will do.
The alternative to mechanic would be to find some way into the fiberglass and gel coat repair world at a marina with slips, but that's a foreign country to me.
2. Do whatever sailing you want and can afford until you have made it into a mechanic job and done it for a number of years with the appropriate training certifications. Then save up and pick out a nice 30+ foot boat.
3. Take your good job and if the marina you started at isn't a good place to live aboard, move to a place that is and work where you live.
Sounds simple enough eh?