My wife has a mobility problem and due to her pride refuses to let me buy her a power chair. But atleast she lets me rent one for her. If you check your phone directory you will find them for short term rent. I pay for 2 weeks around $75. The reason I am suggesting to rent one before you board is because it is a long trip for them to the ship from parking and it is cheaper. Be sure to get the insurance just incase you lose it though. For me it is the best $75 that I will spend on the trip. A power chair just makes it so much more comfortable for the ones who need them and for those that are traveling with them. If you wait till you get on the ship to get one your loved ones have to do a lot of walking to get onboard and then you have to hope they still have (in your case) 2 available. Its not worth the risk. Also if you rent it yourself before you board they get to disembark riding it which will make it more comfortable for them.