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You don't say which cruise line but the newer ones built 1996 onwards and even better the ones built in the last 4 years all have excellent facilities. Most have a reasonable number of cabins with the wider door to take a wheelchair and all the toilet areas have disabled access. There is always the problem at some ports of getting on and off the ship but the staff should help.
If you can get hold of a copy of the Berlitz Guide this has a section dealing with disabled passengers it also rates the ships on disabled facilities,the ships are well suited to wheel-chair users but a lot of the ports are not.As Blyth has said the newer ships better than the older ones but as most are big they can take a lot of getting around,Best of luck.