I don't think anyone means to do this just to discourage you. It's just that many people get into boating (or want to) without understanding the upfront costs and the annual costs. If you look hard (and get lucky), you probably find a decent boat under $15K, but it's likely to be older, most likely an I/O bowrider, and it's going to need more upkeep than a new boat. There are also the usage costs, like gas, launch fees if you trailer (although many of us live places with free boat ramps), purchase of lifejackets, skis/tubes, and that sort of thing. If you plan to keep the boat in a slip or dry storage, there are costs associated with that. And in almost all cases, you have to have a place to store the boat in winter (if that happens where you live); there may or may not be costs associated with that. And then there is insurance, too.
We see lots of folks who buy boats and don't look past the initial purchase price and wind up with back breaking expenses they never anticipated. Nobody here wants to see anyone fall into that.
I think we got into boating pretty inexpensively. We got our boat (19' I/O bowrider) for about $7000. It was probably about 15 years old, and within a year I found out I had to rebuild the transom/decking, add brakes to the trailer (and a controller to my truck), buy a new prop to replace the one I damaged on a wing dam, etc. If I wasn't sufficiently handy to restore the boat myself (with the generous help of the iBoats crew), the boat would have become entirely too expensive for me and my family. I do almost all my own maintenance, and save several hundred dollars annually that way.
Better to know what you're getting into. We've had a ton of fun in our boat and I have never regretted it. But as others have said, it's not cheap, and you're better served knowing that before you go deeply into debt and grow to resent your boat.