If it was empty with no water in it right after pulling out of the lake. I would take a guess exhaust is displacing the fluids ......if it was a while after pulling out the ramp it could have filled with water then drained out ..... I'd be replacing prop seals, shift seal, shaft seal , the entire waterpump and often overlooked the rain bung gaskets . Amazon has great deals on Sierra parts usually and at your door in 2-3 days.