One of the iconic accessories for the Defender is a snorkel. Contrary to popular belief, it is not really designed for water crossings. While it might help in that, it is really designed to simply access cleaner air when driving on dusty trails. For water crossings you have to worry about the air intake, but you have lots of other things to worry about which are lower than the air intake, such as the breather ports in the diff, trans, the electrical components, etc.
It is easy enough to find a snorkel for the ROW Defenders, but finding one for the NAS is very difficult. Mantec made the original aftermarket snorkel for the NAS Defenders and they have been discontinued for years. You could have one fabricated, but to find an original is not easy (especially with all its parts). I traded an extra soft top I didn't need with a guy online that had an original Mantec in excellent condition. Most people install the intake hose from the snorkel by simply dropping it over the engine to get to the air intake filter canister. That would heat soak the intake hose, so I routed it behind the engine and zip tied it away to keep the hose (and air) cooler. I also dropped in a brand new K&N cleanable air filter. The K&N filter for the NAS Defenders are discontinued so they can be hard to find, but I found a NOS (new old stock) in Japan that I imported.
I do believe the snorkel restricts the air flow a bit due to its design. If this bothered you it is easily to unhook the hose to the air filter canister to run your truck in a stock configuration without having to remove the snorkel itself. I am thrilled on how it came out. It is nice to see these hard to find parts being put to good use.