The pa to does have special wheelchairs made to go over sand. I'm sure you could use one of these to better traverse the terrain. I have a photo and will try to upload. The other responder was correct that you do check in one the beach side of the restaurant, but there is a ramp that enters the restaurant thru the bar area, that would accommodate a wheelchair. Let me see if I can post the photos if they sand wheelchairs.
You should call the beachcomber directly to me sure. Check about parking with them. I think you should be able to get a normal wheel chair into the restaurant but would need to go thorough the bar (has a ramp). For power you really need to talk with them.
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