any protection is better than no protection
Unless it breaks in use
Max, pretty sure any water ingress will be via the axle seals. The factory breather must be a one way valve and only lets air out when the diff gets hot! So by adding a breather there is no pressure on the seals.
When I removed the factory breather to do mine when the diff was cold, there was a big hiss of air going back into the diff. The diff must have almost been a vacuum!
If you must go through deep water, you would normally stop and walk the obstacle and prepare your vehicle, cover things, maybe loosen fan belt, maybe block breather in bottom of air cleaner (Max will know more about that as he has experience in water crossings ). Take your time. It is a good idea to wait half an hour or so to let things cool down as it is the sudden shock of the cold water on the diff pulling the temp down. If you can, stay in the shallows so this cooling happens for a while before you go in deep and test the breathers out.
Yes, the danger point for diff ingress is on the exit on the far creek bank, particularly on a long vehicle so close to the centre is a good thing. a 45 degree bank would reduce the height by half at the rear. A well prepared vehicle should have a pocket of air maintained under the hood by forward momentum. Bit of water in the cab isn't a problem in an SR with vinyl floors. al long as my Car PC does not go under!