I have no problem with Jetskis per se, but same as motorcycles a twat can ruin any place where people are just trying to enjoy a quiet moment. You can blast the water as hard as you want and enjoy yourself, but when you're just doing doughnuts on the water for 15 minutes at a time and not moving more than 50m away from the waterside cafe, you're a twat. You're also a twat when you start your bike for 5 minutes before leaving said cafe and never stop revving it through that time, or arrive causing exhaust explosions with your cut off switch, you're the same twat no matter what vehicle you chose to enjoy.
Hell, a twat can even ruin that same quiet place without any kind of noisy vehicle, just by arriving on foot ?