Here is a picture that shows the famous 'wall of water'
On our last ride, I got one of the splash seats and was totally wet to the skin. I went into the bathroom and wrung at least 2 cups of water out of my clothes. I was still wet enough, even after that I was still dripping for the next half hour.
So even if he removes the leg, I would still suggest he wear a poncho - otherwise his clothes could drip on the leg.
If I do end up being the leg keeper, it would help to know, does the ride start and end in the same place?
The ride starts and ends in the same place, but the regular boarding area has a round rotating load and unload area. The boats are moving along with the rotating area, so it is not totally stable and guests have a short time to old and unload.
He should load at the accessible loading area shown in this picture:
As an empty boat goes by, the CM opens the gate (the bar going across the water in the middle of the picture). That allows a boat to enter the area. The the CM raises the bar again, which traps a boat so that it will not move until all the guests have boarded.
For exiting, the CMs do the same thing.
It is not an easy transfer. A wheelchair can be parked next to the boat, but you have to go down into the boat and then over to a seat. The seats are made of plastic and the seats and floors can be wet and slippery because of water coming into the boat. For guests using wheelchairs, once you have gotten into the boat, the CM will move the wheelchair out of the way. You will return to the same dock when your ride is over.
There is LSO a transfer assist device that fits over the side and guests can sit on it and sort of bump down into the boat.
His leg could be left at that boarding area, but would be safer if you or some other member of your party actually serves as the 'leg keeper'.