Yes they are. You can also usually get a disabled cabin that is specifically designed for wheelchair users. Was there a pacticular ship that you had in mind? If so you can check on the Berlitz Guide part of this site how many disabled cabins they have available
The more modern ships are very wheel chair friendly both P&O and RCI offer good facility's at Southampton's port you can book a disabled cabin wide door lower furniture and wet room the excursion desk would help and advise you on suitable shore excursions or ports that have reasonable wheel chair access?