That is actually a water outlet. As standard, your defenders inlet is on the side of the wing. Water can easily get in there, so this y trap is there to let the water back out. The mesh is supposed to work as a kinda one way type of arrangement. But as you say, probably wouldn't stand up to sitting in water. The instructions for my safari snorkel tell you to seal it with silicone. I haven't done that yet, but planned to make some type of cap to go over it, that I could drain off if needed.