I had this same problem and what I ended up doing was going to Lowes and buying a PVC valve. I installed it in the middle at the engine compartment and kee it closed unless water gets in the boat. Then, I can open it and while running, the water drains.
There are aftermarket check valves you can buy but if you've read my posts, you know I'm an elcheapo and look for the McGyver ways around spending my money!...........
At least now, when I stop the boat to swim and everyone is on the back of the platform, the boat doesn't fill up with water!.........:rofl:
Oh, BTW, I have the 97 Challenger