My daughter can't stand at all and we found that the last row of the boat is wider so she can pull her chair up and slide out and then she sits up on the edge of the boat and pulls herself up into her chair.
This is a picture that shows the last row of the boat so that you can imagine what Michigan is describing.
it would be nice, but I can't see it ever happening.
There are a couple of reasons why a wheelchair accessible boat can't be made for Pirates.
The first is the length of the ramp that would be needed.
katieeldr was correct about the slope. For every inch up or down a ramp needs to be 1 foot long. So a 6 inch difference would require a 6 foot long ramp.
The boats would need to be similar to the ones on Small World where the entrance is on the back and and starts out at floor level. A sharp turn is needed to get onto the boat, and then the ramp starts going down and goes all the way down to the front. To get a long enough ramp, wheelchairs would be parked at the very front of the boat.
That would bring the second problem - the drop.
Cheshire Figment and Vickeyrowe mentioned the drop and that it old be difficult to tie down wheelchairs securely. They would need to be tied down really well (at least as well as on buses).
The drop is 15 feet - if you have a stairway in your house imagine how steep that is. Besides the drop and securement, a boat with a ramp would also put the majority of the weight at the front of the boat. I don't know how that would impact going down the drop.
The last part is getting boats back to the boarding area from the Unload area.
After guests get off the boats, guests go up a steep moving ramp to get back to the level of Adventureland ground level. The boats go through a door just a little bigger than the boats and go up to the boarding area.
So, if the did anything to make the boat accessible, it could not stick out or make the boat any larger or it would not fit to get back to the boarding area.