The OP said he was sitting in the water at the end of the course. We don't know what end the other boat was coming from. If the other boat came from the far end he probably didn't see the ski boat at the other end. Technically the course is not in use and the boat with the police officers had every right to enter the course. Once again, this is the big problem, we ASSUME that other boaters know what we consider common courtesy in a slalom course.