What ports? RCCL, Princess, HAL, etc. will often offer limited wheelchair accessible (van or bus with lift) tours in ports where they are available, but often they are NOT available outside of the USA or Canada or the UK. It is generally best to call the Special Services desk of the cruise line (except for Princess, which does not offer this) prior to going on the cruise to find out what is available. Be VERY specific about your needs....they often don't understand that wheelchair accessible does not mean you are fine to take "just a couple steps".
On the other hand, you can often get better tours and cheaper ones arranging them on your own. There are a lot of resources for finding tours in various ports, including this site and the Disabled cruisers ***commercial reference deleted***
If you can transfer to a car or cab, renting a car or taking a cab may be the best and least expensive option.