This is a center face swim snorkle with a self draining valve. I printed the "snokle" and "headband" in 3d Solutech flexible at speed 2400, retraction 0, temp 210, 0.32 layer height. The remaining parts are PLA.
Snorkle: The pictured unit has 2mm wall thickness at the mouth piece but leaked slightly along the tube due to printing defects. I sealed these with silicone. I increased the wall thickness to 3mm and printed but did not assemble another unit.
Valve: To finish the valve body, I infilled one side with cyanoacrylate and then sanded until smooth with a progression of sand papers. For the mechanism, I tried various printed flaps without much success. I think the small channels that remain after printing allow water to infiltate. I eventually settled on small suction cups from the craft store. A 5mm screw will self tap in the center hole and can be tightly forced through the small center hole in the suction cup. You can control the amount of valve tension with the screw. This mechanism is not as good as the thin flap silicone valves on commercial products but does work passably. You may need to occlude the tube as you swim for total clearance.
Assembly: To minimize leaks, there is intentionally 0 clearance. Gather the "snorkle", "tube", and "valve body". Boil some water in a container that will allow you to plunge the end of the "sorkle" into the water. I did this with a coffee cup in the microwave. Immerse the the tube end of the "snorkle" into the hot water, remove, and work the tube into the slot. Do the same with the disassembled "valve body" on the other end. The valve is little trickier and I had to use a screw driver to coax the bead over the valve body. You do not need to heat the PLA of either the valve body or tube.
Headband and Head Piece: I used 1/2 inch elastic from the craft store. You could probable scale the head piece in the Z axis if you want it thicker. This could also probably work in flexible material
Leaks: I had a small amount of leakage at the tube end of the snorkle. I think this was due to printing orientation leaving channels running the length of the tube. I sealed these with silicone. I also had leaks in the walls of the "snorkle" as mentioned above. This was due to tear out of support material, odd printing defects, and probably too few layers. The 3mm version seems better but I reccomend 5 layer wall counts and then sealing as needed.