You might try tying a string to the duct where it attaches to the floor register and then disconnecting it from the register. If you're really lucky, the water will drain out of the duct, into the underbelly. Use the string to retrieve the duct and reattach to the register. If the water drained into the underbelly, drill a few 1/4" holes in the coroplast to let the water drain onto the ground. The coroplast is at most 1/4" thick, so don't drill more than that so you don't contact anything else.