When I had a boat shop we had a 400 gal test tank built. It was designed to allow big outboards and stern drives to be run under load. A stern drive will run quite happily if the starting water level is just up to the bottom of the pump, even the gen IIs. When we needed to empty the tank, we'd put a big block bravo drive in. Drop the hammer hard and the tank was empty in about 4 seconds!