We have a boat. It's awesome to have. 18.5 Glastron Fish/Ski combo, 190hp volvo penta. Gas at the marina is expensive - the only option where we spend a lot of time is 91 octane no ethanol. $4.54 a gallon last week. That translates easily into $100 for an afternoon of tubing and skiing. Putting that into perspective though, it costs a hell of a lot more for a lot of other recreational activities - 4 people going to a concert, or going to a football game, etc. In fact, those activities are a hell of a lot more expensive than boating.
I really don't think it's that expensive overall, but I have the space at home to store it & don't have to pay to keep it at a marina. Launching a boat under 25 feet, despite what someone said above, is pretty easy, though I've seen some people at the boat launches really struggle. The general problem is that they can't back a trailer down the ramp - uncoordinated idiots. (Practice in a parking lot somewhere so you don't annoy people who have to wait for your sorry ass. - Easiest way is hand on the bottom of the steering wheel, hand goes in the direction you want the trailer to go.) Some service can be a little expensive; I just dropped mine off Saturday for an oil change and to have the usual service done - checking the lower unit, changing the oil/grease where it's needed. That'll run me about $250. (And as a courtesy, they'll regrease the hubs on my trailer, which saves me a bit of time.) The water pump just started leaking. The marina said I'd probably make it through a week of boating, but after that, I'd need to take care of it. If I had them do it, I'm guessing a couple hundred dollars. I found a kit for replacing the impeller and o-rings etc., on sale for under $20. Last year, my starter went. It might have been a do-it-yourself job, but fortunately, I had the marina do it. From an earlier service, they had pulled it and reused the same bolts. They should have replaced the copper clad bolts, but didn't. The result - the bolts started twisting without the threads giving - my block had to be heated to get those bolts out (add a couple hours to the job) without snapping the bolts and needing to drill them out. Drilling them out would have meant the entire engine had to be pulled, changing what could have been a $200 job without problems, that was a $400 job with problems, into a small fortune.
Overall, I'm quite happy with it. It's got plenty of power for pulling skiers and tubers. On relatively calm water, it runs around 50mph with an aluminum prop. I could get more speed out of it with a stainless steel prop, but don't find that necessary.
Extra equipment isn't as expensive as someone above said. We have enough of those tacky orange life vest things for the capacity of the boat; they cost about $8 each. We also have the much nicer vests, especially since we tube and stuff - since I'm good at bargain shopping, I can get them fairly cheap ($20ish each) - just bought two of them to replace the yucky ones in the kayaks. They were on sale BOGO. I've gone overboard (pun not intended) with purchasing safety equipment - I'm supposed to have a fire extinguisher, I have 2. I'm supposed to have flairs or other visible signalling device - I have both and twice as many flairs as necessary. 1 throwable cushion - I have 4. Whistle as a back-up to the horn, etc. And still, I'm no where near $400. In NY, registration for the trailer is about $20 a year, and registration for the boat is good for 3 years (I think) for somewhere around $100. Other states vary significantly on this. No "license" is required, though you have to take a course and have a certificate before operating a pwc. I'd prefer that people DID have to take a course before boating, though I haven't run across too many morons on the water. Neat fact I learned from one of those courses: it's legal in NY to be operating a boat while drinking a beer; I never would have suspected that. Though, you can still get a dwi if you're intoxicated.
*note: things I ran across last week at our favorite impromptu social gathering/party spot (a shallow sandy cove, warm water, few weeds, 75% protected by a nasty shoal) - a pickup truck tool box retrofitted into a floating pontoon charcoal grill capable of cooking enough bbq chicken or hamburgers for a few dozen people. Also, a floating beer pong table (same cove.)