As it's a diesel all it needs to run is to get air and fuel ... make sure the snorkel is sealed properly, and similarly the fuel system. The rest of the elctrics usually aren't harmed too much by water and the engine will continue to run.
Having said that ... I spray the fuse box, when I remember to, with a water inhibitor 'cos I have a tin! Mostly I don't remember to ...
The main issue from above is all the other bits that might get water into them, axles, gearboxes, engine, etc.
So easiest is to fit axle breathers and route them up into the snorkel. Make sure all axle seals and bearings are in good fettle, similarly the diff seals .. if anywhere's letting any oil out, water might also get in!
Same with the gearboxes, add longer breathers and check the seals on input/outputs from the transfer box. Keep the gearchange tops clean, greased inside and the rubber shrouds tight to the gearbox tops and gear change levers .. it makes a big difference when water gets into the cabin!
On the engine I also have a timing chest breather, fit wading plugs (and remove them regularly for draining!) again check for leaks and replace leaking pulley seals as and when you can. Keep oil leaks in check as they'll let water in too .. Keep the dipstick pressed in, and tightened/sealed properly at it's base.
Don't try to cram all the breathers into the snorkel, get a manifold and route them to that, high in the engine bay, then a single pipe into the snorkel.
I also use the water dispeller/inhibitor on the injector pump and moving parts, throttle linkages etc Dunno if it does anything, but it feels like it should!
One thing I do now is to not try and stop water getting into the body .. My old Disco floated away, with me in it, a few years ago after I repaired a few holes trying to stop it from letting water in. I let the water in and let it out quickly too ...
