DR Yachting in Athens, Greece is a full service "disabled yachting" company. I have read there whole site and this a small excerpt from the massive list of how much this company has done for people with disabilities to enjoy a top notch sailing vacation in Greece.
"In order to offer the joys of sailing and relaxed and happy vacations to disabled guests, we made the following adjustments to our sailing yachts:
We created a “catwalk” sized 2.5m long and 90cm wide, with 10cm heightened borders (in order to avoid any accidents while a wheelchair is on its way in or out). There is a rail on one side that supports walking persons." (read more)
Here is their site, with many pictures, and the available itineraries.