If plugs are fouling it's another problem.
And the operating conditions present challenges that add complexity. Lack of maintenance can be a biggie ("it's another problem").
Not just Rotax turning at 7,000 RPM, it's the marine environment. Lots of things can happen. For example, lets say you hit a wake and flipped upside down briefly and some crankcase oil vents through to the intake, enters the cylinder and glazes onto the hot plug insulator causing contamination? Or the ski sits around for a little too long and the fuel gets a little stale or water, and glazes the insulator during a hard run your average car is unlikely to experience. Who are that nobody that rides WOT across the lake like there's no tomorrow, does he drive his car that way too? I doubt it, thus the root cause might even be the nut behind the wheel.
Please don't claim it's a car, this is misleading b/c generally speaking, a marinized engine runs with a more conservative tune using a colder plug than automotive engines, less aggressive ignition timing and a richer air/fuel mixture because the marine environment is considerably (not slightly) harsher in comparison. Marine engines are burning a heck of a lot more fuel (measured in GPH as opposed to MPG) than an automobile engine loafing down the highway @ 1800 RPM. This translates to considerably more heat.
High duty cycle with no coasting up to stop lights or down hills, on and on.
The marine engine is a beefed up version in comparison and the 4-tec as far as I know doesn't benefit from an O2 sensor in the exhaust (it's a wet exhaust) as used by automotive computers to monitor air/fuel stoichiometry. For marine FI engines at least up until recently, there's just a simple lookup table used to produce a conservative air/fuel ratio approximation based on throttle angle, RPM and intake manifold vacuum + added safety of slightly richer mixture to compensate for (un?)anticipated harsh conditions.
So on top of high power requirements, burning one heck of a lot more fuel and as a result making a heck of a lot more heat, the air/fuel ratio is considerably richer for the purpose of maintaining a reasonable safety factor.
Add this all up, and 200 hours reasonably represents IMO, an accomplishment.