You could try and google the ports your calling at then try the tourist info site this may have local taxi or coach tours that meet your needs best of luc.
Hello I think you are making a very good point as America prides itself on provision for handicapped people and many access measures have been taken there so if you are indeed talking about, as you have indicated, P & O and North America then provision should be available for wheelchairs and their users on the transport there. If you are having difficulty talk to P & O and if their answer is negative than challenge them because in this day and age they are dragging their feet. Hope that answer suits you....Heathe.
On P&O cruises I have been on, there has often been wheel chair accessible coaches available at some ports. Depends a lot on availabilty in the local area and they get booked up quickly. Usually EU countries are OK.