Not a problem, just so they fit on the site.
You could also park the jetski and the ATV (I'm assuming the jetski is on a trailer) up at the front lot. There are plenty of parking spots with trees/light posts and fence posts nearby. You could lock it up to an object in the parking lot and park the ATV so it is blocking moving the jetski anywhere and then they would be out of your way at your campsite.
You can't use the Segways in the parks, but you can use them around Fort Wilderness. Just need to lock them up when parked.