Diff Breathers
When a hot axle hits cold water it cools rapidly. This has the effect of lowering the air pressure inside the axle and the diff housing, causing air to be drawn in via the diff breathers. Extending your diff breathers higher up the chassis (standard on most Landcruisers) with the aid of plastic tubing is a "must do" for regular water goers. However, it is no guarantee that water will not enter your diff. The sudden reduction in air pressure inside your diff can still cause water to be drawn in via your axle seals if the diff breathers do not equalise the pressure quick enough. This is why its still necessary to allow the vehicle time to cool down, even when extended diff breathers are fitted.