How do you get these used jet skis/boats to your local boat ramp? Do these used jet skis/boats have wings? Do they run on air? Where do you keep them? These questions cost money. Those engines will be a lot more expensive to fix than a broken ski.
The idea that boating can be cheaper than skiing is silly. The only way boating is cheaper is if you never use the boat after you buy it.
So how many boats have you owned and maintained to be this font of knowledge on how much boating costs?
I've owned boats myself for 10 years. My family moved from Michigan to Colorado around the time I was born, so we were the weird people in Colorado that actually owned a boat- a nifty 19" Seadoo runabout.
For me personally and my family growing up, boating has always been the cheaper activity.
In answer to your first question, they get towed the ten minutes to the ramp. My tow vehicle gets 19mpg towing. Most people live much closer to a lake with a ramp than they do to their ski hill, and even when taking the gas milage hit, they spend less in gas getting to the ramp. This is geographically dependent, but for many/most, the lake is closer than the transportation costs related to skiing activities.
They run on gas. Two have 5 gallon tanks, my sitdown has a 10 tank. On all, that is good for a full day of use on all. At $3.50 a gallon, that means burning $70 of fuel in a day. Split by at least three people to be running the tanks dry in a day, that is $23 a person. Seems to be cheaper than skiing. The typical gas tank on a runabout ski boat is 30-40 gallons, and in ours, that would last a weekend. Divide that by the 4-6 people hanging out in the boat taking turns skiing and wakeboarding off the back, and again, this is very affordable.
I keep them parked at my house. You see, many of us have homes. These homes have garages and driveways. You can park things in them. If I didn't happen to have room, I could pay $20 a month to park someplace else.
Engines will be more expensive to fix than a broken ski- NOPE! For one, they last much longer than a pair of skis because they just don't see the daily use that a car sees. Rebuilding a Ford or Chevy small block, which comprises the engines most commonly found in runabout ski boats, will cost about the price of a set of Kastles. But that engine can be expected to last 20 years.
In the ten years of owning my rig, I've had to rebuild 2 jet ski engines. This is what the top notch high volume rebuilder charges, again, a pair of on-sale Kastles. The last engine I have able to source the parts and do the work myself for about $350. So, over ten years, $70 a year for engine work. Engines last longer than skis and boots.
Does stuff break? Sure, but there just isn't a whole lot of stuff on a boat or ski that is terribly complex, and 90% of issues are electrical caused by condensation and vibration.
The big investment in boating is the boat. I'm just pointing out that there are a ton of people doing it with equipment that they paid under $5k for, and once you own the boat, the rest is cheaper than skiing. Own the boat for a few years, and even factoring in the cost of the boat it is still a cheaper hobby than buying a season pass every year.
Its not that boating is cheap. It is not. But, it can and is done cheaper than resort skiing by a lot of people.