For one they have the metal shield housing and they silicone them down. Two, they are not meant to "stay" in the deep-water. They are only meant to go from the landing craft to the beach. I know as I was in the Marines and that is how they where driven. Also any craft that has gone into deep water is immediately sent to maintenance (as soon as hostile actions is over) and gone through. Again I was in "motor transport" and had to perform this function many times. At no time is it recommended to drive your "HMMWV's" continuously in deep water. Read the Operators manual.
As far as there being a different "front seal, torque convertor, or pump" there is not. The only thing you can buy different is a silicone front seal and that is aftermarket racing only...
This has been my experience with civilian Blazers and Suburbans as well as Willys Jeeps as well.
They can keep water and mud out during a quick ford but if you get stuck for long, all the heat in the axles and engine being quenched in cold water of mud creates a vaccum in them that draw water and even mud in past the seals.
My roommate destroyed the 350 in his 72 K5 doing just that. The intake didn't go under water and was clean as a whistle ditto the valve covers and pcv valve yet when he got stuck and stalled the engine deep in the mud pit, his crankcase filled with silt.
Looked like chocolate milk when we got it out and drained the oil pan.
Back in my mud running days I figured it cost me $300 to replace the axle seals and rear brake shoes and repack bearings, inner front grease seals, sometimes front pads, every time I went through the mud pit.
Therefore, when I had the urge, I would go through it numerous times back to back till I got it out of my system since it was $300 bucks when I got home no matter what.
As for the op question about fording; On my 1984 K5 I attached an aluminum flexible dryer vent hose between the snout of the air cleaner and the firewall where I cut a 4 inch hole and stuffed the stock air flange (the plastic ring that holds the intake hose in the radiator core support).
It wasn't water tight but it kept splash out.
Drew air from just below the windshield so I would have to be hood deep before it drown.
Of course that was a gas engine.
When it drown a few times I just shut it down and pulled the plugs. A cranked till the cylinders pumped the water out.
The repercussions for a diesel might not be as rosy.