Most 4x4s can be driven in water that is axle-deep without taking special precautions. (Max. wading depth is about 20 inches.) When the water is deeper, you need to know where your engine’s air intake and engine computer are located and don’t allow water to enter. Switch off headlights and allow them to cool, as sudden contact with cold water will cause the glass to crack.
If you place a tarp across the front of your vehicle prior to entering very deep water, you will minimize the water entering the engine bay by creating a bow wave, so long as you maintain a brisk forward momentum. The result: less water will be sprayed over the ignition system by the radiator fan and less chance of water entering the air intake. Do not outrun the bow wave.
Another thing to note: Crossing a body of water has more to do with the technique you use to get across than it does with the kind of equipment you have. A paced, controlled, and planned-out method of driving is far more important than a snorkel.
One technique is called the "bow wave." Check it out:
This driver is moving at a controlled, yet moderately aggressive pace that creates a wave in front of his vehicle. He has a pocket of air within the engine compartment thanks to the displacement of water. His speed is not too fast, and not too slow.
Equipment will never take the place of knowledge, experience, and decisive driving.
When crossing shallow streams, drive slow and steady to create a small bow wave in front of your bumper that will reduce the height of the water behind the bumper and keep the water away from the air intake and electronics. Select low range and first gear, and keep steering straight.
When crossing fast-flowing shallow streams, cross at an angle and drive slightly upstream. This presents a smaller surface area and lessens the force of the stream on the vehicle. (Never cross fast-flowing deep streams, as your vehicle can be swept away.)
Apply your brakes several times after crossing water or deep mud to dry them out.