Morgsmom is correct. There is no chair lift in the pool on The One Deck. For the YC beach area, while there is a hill to get down to the beach, but the Cay does have "beach" wheelchairs with the oversize tires. When on board, I would remind the concierge to make sure a beach chair is reserved for you. One YC guest used on at the YC beach, but had to be assisted up and down by a few of the lifeguard/attendants at the beach.
It would help if you outline your needs in the Special Needs form. You can detail the type of wheelchair you are bringing or need on board, among other things. Here is a pdf of the form attached.
Special-Needs-ENG-BASIC FORM.pdf