My take on it , is the drain is only designed for the exact same conditions as the factory airbox air intake, so i doubted its ability to keep water out when "submerged", (mine did not look like it would keep water out, and is located directly above the LF tyre), IMO ,just not worth risking it on a 14yr old bit of rubber located at 1M high.
That said, be interesting to see what Nissan instructions are with regard to this plug, such as when you buy a genuine factory snorkel, also do they say anything about maximum water depth after snorkel fitment ? (i know some people complain of the rivets holding the nissan badge on the side leaking water)
BTW, In the 4.5L set up, the drain tube that this plug is connected to, makes getting changing the aircleaner awkward (you have to remove airbox from its mounts), its very hard when you are trying to remove it and NOT disturb your inner gaurd silicone sealant (keeping your intake airtight) , blocking the drain off, also allows you to shorten the tube, so now i can change aircleaners easy without disturbing the seal/s.
Maybe its a 4.5L thing?