I am not 100% sure. I don't think it is a drain tube. What I think it is: the seat compartment is in 2 pieces, one piece is the sides and bottom, the other piece is the top. There is drain hole in the top that is supposed to let water go down behind the seat compartment and just go overboard. In the seat compartment you can see a vertical part molded in (circled in blue in my pic)
The entire seat compartment must have a flange on the top so that the seat top has something to rest on. If the seat top and vertical part are not aligned the hole in the seat top will be sitting on the flange of the vertical part. And water will not be allowed to go down the outside back of the seat compartment but instead will flow into the seat compartment, probably under the cushion itself.
You can see in the 2 pictures here, one drain hole is completely open, the other is partially blocked with the top part of the seat compartment. If you cannot stick a pencil in the hole, then it is blocked. And when the channel fills with water it will overflow into the seat compartment. The picture someone posted earlier looked completely blocked to me.
If you can move them around and get the holes aligned you can drive a sheet metal screw next to the hole which keeps them aligned. You may also be able to just drill a new hole in the seat compartment flange, but without seeing what the backside of the seat compartment actually is I would not do it. However if you cannot get them aligned that may be the only option. Just make sure you don't drill a hole that lets water get into the seat compartment, that will make your problem way worse. The hole has to go into that vertical part molded into the compartment (the blue circle).
Proceed with caution, I am only guessing since I have not taken the seat apart.