Okay, you made look... again...I bought the Aqualung dry snorkel you describe, and got it because it has a GoPro attachment on the bottom of the snorkel. I took it into the river last summer with my GoPro attached, and it was fine as long as I stayed on the surface. Then I surface dived down to anpbout 23 feet, and as I got to the bottom the snorkel tried to suck my tongue out into the tube--I didn't like that, and spit out the snorkel while on the bottom. That messed up the video a bit.
When you completely seal the snorkel, the tube is then exposed to the same treasure differentials as any closed system, and it there is an opening, it tries to fill it to equalize the pressure. So you can say that there is now a thing called a "Snorkel Squeeze" because of these dry snorkels.
I took this snorkel home, heated it with hot water, and took the top off. I then evaluated it without the top, and found that the non-return is not as well-designed as their Impulse snorkel, and leaves some water in the bottom when cleared. I removed it b y closing the top and blowing, which is a good way to clear these snorkels of small amounts of water. Please look at the VSS website link above for more comments on many different snorkels.
John
I find no Aqualung snorkels with any GoPro attachment on them. Maybe someone other than Aqualung?