Most of it is the way they are driven – go in slow, keep up a steady speed that creates a bow wave and it will usually keep the water out of the vitals. Too many people charge in and flood everything – modern electrics are more waterproof than they used to be, but water up the air intake will stop even a Toyota Land Crusher, fine motors though they are.
I've taken a mk 1 disco through water breaking over the bonnet with no mods beyond putting in the cam belt and bell housing drain plugs – but then we didn't have to stop, that really would have been bad news. Mk 1 discos and defenders are virtually the same under the skin, most of the drive line and suspension is interchangeable, its the height and tail overhang that make the main difference. Plus discoveries don't let in so much rain and you can hear yourself think.