When you're offroading it often becomes necessary to cross water. Knowing how to drive through deep water (called forging) can allow you to successfully get through without getting stuck or hydrolocking your engine.
Driving through a puddle is one thing. Driving through a river or pond is something completely different. Not impossible, just different. You can drive very fast through shallow water (less than a few inches deep) and not have to worry about hydrolocking the engine or temporarily shorting the electronics.
Once you get a foot or more deep you need to slow down. The deeper the water the slower you should go. In very deep water (more than a couple of feet) you'll want to drive about 3 or 5 mph in low gear, trying not to make a wake. Never drive in water deeper than your airbox unless you have a snorkel.
When you drive fast through a puddle and see a wall of water coming up on the outside of the tires you must remember that the same amount of water is splashing up on the inside of the tires, usually finding its way to the engine compartment and subsequently into the airbox. Get too much water in the air box and you risk serious damage by having even a small amount of water getting into the engine.
Jeeps, while very capable in water, are not boats. You can't go flying down the trail at 20mph (a fast speed when wheeling, especially when heading into water) and slam into a "puddle" that's a foot deep. You'll force massive amounts of water to fly up under the hood and flood the air box.
The airbox is designed so that if reasonable amounts of water get into the airbox it does not make it into the engine. What I saw in his video was neither a reasonable speed nor a reasonable amount of water going under the hood.