Another piece of de rigueur equipment that serves two purposes. Firstly; to allow deep water wading without drawing water into the engine, thus avoiding the otherwise consequential dire outcomes. The maximum wading depth of a standard Defender is only 500 mm (top of the wheel rim roughly) as the standard air intake is set on the side of the wing. Best also to extend the various axle, gearbox and engine breathers to prevent water ingress. Good advice when confronted with a water crossing is to wade it first; this, however, does not take account of the various creatures that could be present and would wish you harm. I devised a depth finding system of a weight attached to a crab line with floats (corks) set at intervals to give an indication of depth.
Also bear in mind that the ECU is located in the driver's seat box making it susceptible to water should it be deeper than the box top - for seroius deep water work best to extend ECU to higher position as we did.
Secondly; to allow clean air into the air intake. Many African roads are dirt tracks and other vehicles will kick up thick dust. This dust is densest low down and thins higher up. With a snorkel there is more chance of getting cleaner air into the engine without having to clean the air filter every day: mine never required cleaning as a matter of urgency. With the Safari snorkel, in particularly dusty conditions, the intake scoop can be reversed so that little or no dust enters without any real affect on performance. For sometime I ran mine at right angles to reduce the intake of butterflies that can quickly clog the air filter.