My bilge pumps both exit from the side of the hull. I have non-return a valve between the electric pump and the through hull, and a ball valve, which I turn off at sea. The pump makes enough noise that I'll hear it and can open the valve if it comes on
Incidentally, as bilgewater tends to be less than pristine - at least in my boat! - I sealed a 2" bit of plastic pipe (WC overflow, I think) into the skin fitting so the water doesn't stain my nice new paint.