Pacific gas and electric PGE.COM has a deal that helps pay for sealing ductwork from the inside. Its an energy efficiency thing but I believe that the substance they use is water proof. Perhaps you can you this to seal the duct work. A bigger problem for you is why the water is getteing under the slab in the first place. Perhaps you rain gutter down spouts are plugged up, or perhaps they terminate too close to your home. I would suggest remedying this probllem asap to prevent mold infestation.