Sounds like the seal in the pump shoe has blown out since you say the impeller has minimal clearance and no damage (make sure you check, clearance should be around 2-4thou). Remove the pump shoe (clean it) and sikaflex all areas that connect with the bottom of the ski, including under the pump ring. What is happening to your ski is cavitation. You will have to remove the scoop grate with 4 x 10mm bolts and pump shoe with 4 x 12mm bolts. Make sure when you re-assemble you but the long 10mm bolts to the front of the scoop grate.