None of them are water tight. Your axles, tranny, and t-case all have breather tubes. Manuals are better about keeping water out than autos. It's always a good idea to change your fluid after deep or long water crossings.
4x4 -101, you should wait for your vehicle to cool down before performing a water crossing. If you go in hot, the air in your axles, etc.. will cool down and compress. If there is water at the breather tubes there is a posibility it will suck in water.
'03 TJ Sport
/ D30/Ford9 / 35x12.5x15 KM2 / D window black steel 15x8 wheels / 3" custom long arm lift / SYE & 1350 DS / JKS BL / 1" MML / Smittybilt's front bumper, Tube fenders, Rocker guards, and rear bumper / High Rock Tire Carrier