When on the hard, I take the speed/temperature sensor (through-hull type) out. I use that nice big hole in the bottom the run an extension cord into the boat. That way I can run power tools/lights/etc. (I also used it to flush the antifreeze out of the water tanks, using the siphon technique I posted about a couple years ago.)
Well, I left the sensor/plug out when the marina launched the boat.
Fortuantely they are professionals and check the bilges after launching. Or maybe they heard the rushing-water sound that a small gyser of water makes.
Great minds discuss ideas;
Average minds discuss events;
Small minds discuss people.
The best minds discuss sailing. I don't know why. It's a mystery!