Just a thought I never tried this. What about dye in the oil, run it, and as soon as you pull the boat out look for residual with a UV light? I know the water will wash away the oil but there has to be some trace of it, right?

Pull the drain plug enough to see if there is any water, if no water it might be leaking somewhere but no problem-o, can wait and see if it shows up later

The bilge in this boat is totally dry, no gear lube, water, or anything.. The gear lube looked like new when I recently changed it, so there's no water intrusion.