Algae?...
Brian, it's usually algae growth the ends up stopping it up. What I did with mine and do it now and then, is to take a water hose without the nozzle on it. Hold it up tight to the drain plug area and have someone turn it on. That should blow it clear, unless you've had something fall into it like a rock or bottle cap.
You are right, it does drain into the bottom of the ski locker..........:cheers: