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I can't answer about a guarantee. My husband had a stroke a few years ago and uses a powerchair. We always book an accessible cabin with a balcony. Bathroom shower in accessible cabins have a fold down shower seat. Also accessible cabins have more room and wider doors, which is needed for a scooter or wheel chair. Since some of the most walking you need to do is on the ramp during embarkation, request wheel chair assistance. I would recommend renting a scooter for use on board and getting off the ship at the ports you visit. A scooter is easier to operate if you're not used to a power chair. There are several companies that rent scooters for use on cruises - they are delivered to your cabin and not used during embarkation or disembarkation.