Generally speaking if I am using an ECV and I see rain coming I will head for shelter, but what to do if there is no shelter available. I will have at least one poncho with me. And I don't want rain to keep me out of the parks. I have severe arthritis in one knee.
I use another poncho for the scooter steering column. It is important to keep this dry. I fold down the seat and keep it covered along with the column when not seating on scooter. It helps the seat stay a little dryer.
I just wanted to comment on your post, I stayed at BWI out last 2 Trips and used the boats to both Epcot & DHS. It is really easy to load and unload on the Friendship boats.
I have read before about using a shower cap to cover the control panel. I was lucky we had no rain our last 2 trips hope I am lucky during my next one.
The scooter I recently rented from Walker Mobility came with a small plastic cap to cover the controls. I also had a rain poncho I put over the steering mechanism to keep it dry.