the exhaust pipe in a 4 stroke multiple cylinder engine has enough pressure to move the air out of the exhaust pipe anyway - I dont get why people have this false notion. Its only valid on 2 strokes where the exhaust actually gets sucked back into the cylinder.
The engine shutoff happens when the electrical system goes damp from the high level of water. If it does, dont kill the battery, remove and dewater the distributor cap with WD40 and apply loads of petroleum jelly for water insulation, hard core (read insane) offroaders also use contraceptive condoms with petroleum jelly on old distributor and HT lead type ignition systems.