We have an 18' Lund Alaskan. That is an aluminum fishing boat. Ours is a 1985, we bought it in... 2000 when I was 10 years old, my brothers were 7 and 12. Total we probably have about $15,000 into it over the past 16 years. That includes everything--purchase price, new floor, rebuild of the engine, trailer maintenance, new seats, paint, storage*, insurance, tags, etc.
Could we have rented cheaper? Maybe. It would have been very close--some years we used it 3 times a week for the entire summer, some years I'd be shocked if we used it for 3 days. But we didn't have to plan to use this one, we knew we had it. We got to learn as young kids how to paint, do minor engine maintenance, how to hook up a boat battery, how to take a boat out and do the safety checklist, how to moor it outside, how to... Sure, all of these things could be learned using a rental, but we had something to show for it. A wonderful bonding experience with my brothers and my dad.
*We stored it for the first 12-13 years at my grandpa's vacation home. Since he sold that, we've been paying storage--probably about $400 for the year. Because of that, my costs may be too low, but the only thing I'd be underestimating are the storage fees. Additionally, we didn't plan on storing it this long. Since grandpa sold his place, dad has been trying to buy a place of his own, but the options are few and far between.
Be realistic. Will you get the use of it? My dad is looking at a new boat--probably will spend $20k to $30k on the purchase price. But he is planning to keep it nearly until he dies (he's 62; his parents lived to 87 and 91) and us kids will get free reign with it--3 kids, and hopefully grandkids soon. My grandpa always said that his vacation home and boat were the best investment he ever made. I can tell you he may never have seen any positive returns on it (I'm sure he didn't), but he was right. I'm very good friends with my siblings and my cousins and my aunts and uncles because of it. And he could afford it, so what did it matter? He and my grandma got immeasurable joy out of seeing his family enjoying something he provided. My dad now wants to do the same. I'm happy for that.
Boats are wonderful. Who cares if it isn't a great investment, sometimes the return is hiding between the lines.
EDIT: I should note that we're talking about Lake Michigan here, which is why Dad is ok spending that much. A 16' runabout speedboat isn't great--we know, we've done it. If you're on an inland lake you can get away with something much smaller/lighter/cheaper (not as well built). A 12 foot boat isn't going to work for more than tooling around the cove (we had those too--they're great for that; those are essentially free if you can store it somewhere).