Yep, Bill has a great picture and a good descrption.
Pirates at WDW has the entrance in one building on ground floor. The exit is in another building, in the basement. When you get off the boats at the exit, the boat goes up a steep ramped area with just enough clearance for the boat to get thru (no passengers). So, it's not possible to get on at the exit like it is for some other attractions.
Because the entrance and the exit are so far apart, they prefer people to switch from their own wheelchair to one of the attraction wheelchairs at the entrance to the queue. That way, they don't have to move your wheelchair from the boarding area to the exit. What they usually do is just put the 'loaner' wheelchair into the back of the boat and it travels thru the ride with you.
CMs will sometimes move a wheelchair from the boarding area to the exit, but if you do that, you take a chance that you may arrive at the exit before your wheelchair does. We did it once and it worked out, but other people have posted that they arrived at the exit and their wheelchair was not waiting for them.
Also, Bill's picture show to height of the boat side. It's kind of like transferring over the side of a bathtub and onto a bath seat about 6 inches lower than the side. If he can help you, you might be able to do it, but, it is not an easy transfer. If he can't help, keep in mind that it is much easier to lift someone in than to lift them out. My DD is only 5 feet tall and is 86 pounds, but this is one of the harder rides we put her on.
Also, the way they are loading people with wheelchairs (or were the last time we rode) you get in on the side shown on Bill's picture and off on the same side. The 'fences' that the picture shows are to separate out the groups of riders into individual rows. The back row (the one shown in the picture) has a little more room for transferring. The rows farther forward have 'fences' pretty close together.
One final thing that Bill's picture shows well is that the seat is pretty shallow. If you can't get the person all the way back in the seat and/or if they are sitting forward on their coccyx, not much of the butt is in the seat.