I actually think private lake course skiing "might" be harder on the engine/driveline than the recreational Joe who is cruising lakes and pulling kids around. It depends whats harder on the motor, lots of idle time with relatively short periods of 3500+ RPMs (think cop cars) or long periods of cruising at 2500-3000 rpms (your highway commuter).
Just to try and put the hours in perspective. My 2010 Ram has ~1800 hours. I have almost 70k miles which is an average of 12-15k/year. My truck probably averages below 2000 rpms which is barely doing any work on the engine/tranny and despite what my Ford and Chevy friends say, the engine/drivetrain should go 4-5k hours without too much trouble. My parents first MC, we put ~1600 hours in 8 years (Almost as much running time as my truck that I drive daily) but it was LOTS of pulling me and friends around a big lake on beginner skis, kneeboards, cruising, etc. And that boat was super reliable up until we got a newer one (and had solid even compression when we sold it).
Now lets take a look at a boat that primarily pulls 34-36 mph skiers. The boat is started up and doesn't really get a chance to warm-up before your into the course and pushing 4k rpms. I wouldn't hammer my truck engine like that on my way to work in the AM but I don't think twice about it on my boat... Yes its harder to put 1800 hours on a boat doing this unless your running a ski school or its a communal boat but those 1800 hours are pretty hard.
Moral of the story is high hours don't scare me, but its all relative to how the boat was used as 1800 hours of 36 mph slalom is not the same as 1800 hours of towing tubes even if identical boats were maintained exactly the same.