Ahh a very very good point. Now if the thing was running the air intake would definitely draw in water. I don't know if it would kill the unit completely, there are no moving parts in them except the fan, which would be inside the vehicle and probably isolated from the water.
At a guess, I would suggest that once the burning chamber had been cleaned out (maybe a new glow plug too) the unit would come back to life. I had located my exhaust in a silly place on my last van and the end of it often got clogged by mud which got a bit baked into near concrete hardness but it never failed even when splashing through very deep puddles until the pipe was completely sealed by the blockage. A quick dig with a screwdriver (like cleaning your ears with a cotton bud) and she was right as rain until the next time. The intake was inside the van, only the exhaust was vented to the outside via a wheel arch, the unit was actually fitted on it's side in the back of the van. In a transit, I had mounted it high up (upside down) and vented the exhaust through the roof via a plumbing fitment for the water supply to a household immersion heater hot water tank. I used a 90° bend type with a short bit of 22mm copper tubing soldered into it as a connection for the flexi-pipe from the unit to attach to. This was mounted on the roof of the transit and a little wire cage put around it to make sure no-one accidentaly touched it when it was hot - though it would serve them right if they had, they shouldn't have been anywhere near my roof. As I said earlier, mounting the unit up high lessens it's efficiency unless you have an air intake for the fan low down because hot air rises and your feet will still be cold when the rheostat thinks they are nice and cosy.
