As someone whose parents own a marina, I can say with some authority that
1. there is no "inexpensive" boat to own, unless it's a small sailboat.
2. there is no "cheap boat that can be fixed up for cheap," unless it's a small sailboat (and even then, not that cheap).
3. it's alot harder to go junkyard-diving for parts for a boat than for a car
4. there are alot less boats than cars out there, and alot less used parts. Most "used" parts that come off boats came off because they were no good.
Aside from that, when your GRM beater car blows up an engine you just end up on the side of the road. When its roof leaks, you just end up with some water in the floorboard.
On your GRM boat, when the engine blows up you're sitting in the middle of the bay and getting a towboat to come get you isn't cheap, and takes many hours. And when it springs a leak, you may sink.
If you do get a "GRM" boat, make sure you get a contract with a tow company (like BoatUS). It's like AAA for the water, but far more important if you have an old boat.
And get a kicker, definitely.