Just read this through, quite interesting - did anyone ever make a snorkel for the P38 then?
LR quote a wading depth of 500mm for all their vehicles (540mm for the P38 in 'offroad' EAS setting, 400mm for Freelanders), which seems pretty low to me - only 20" (21.5") of water?
We took my mates Disco 1 V8 with no breathing mods and on standard suspension through a water run in Wales that was about 20-25 metres long and up to top-of-the-bonnet level with the bow wave with no problems. No water ingress, no water in the engine bay, etc. Obviously if you stop then the engine bay will get flooded, but that's the same in any vehcile. As long as you keep moving and keep a bow wave going, I can't see why a P38 would be any different?
I'd take my old Classic through water up to a metre deep and would take a P38 through the same I reckon. So pretty much any flood you're likely to find on a UK road!