You'll have to take my comments with a huge grain of salt, because I don't have hard-core off road experience. That said, I have seen the Psychotty units up close enough to tell you that they take suction from just below the plastic grill that runs the width of the truck right below your windshield. So if you don't bring water over the hood, it should make a good snorkel, but, if you ever get a surge that washes the hood up to the base of the windshield, then it will become a funnel instead of a snorkel. In addition, there is a small amount of air that enters the Pyschotty near where the hose from the turbo currently connects to your factory air-box - I think there were some minor problems with filter collapsing until this feature was added. But you'd be in pretty deep water, to within 6 inches of the hood, for it to enter here. Also be aware that the Psychotty also increases in-cab noise; but posts on this board seem to like that feature by about 10 to 1 or more.
I would urge caution using your Cummins in water deep enough for this to be an issue, unless you are relatively wealthy. You probably already know this, but, I think this engine would be less forgiving of deep water for two reasons - high compression ratio diesels pretty much self-destruct if the intake is flooded (so I've heard), and secondly, I know the block is a massive piece of iron, so sudden temperature changes by quenching in water can't be good. Big $$ difference between making a misjudgement with this beast as compared to making the same misjudgement with a gasoline engine.