In the Bleak Hollow on the Isle of Thunder, if you go where Taran Zhu, Taoshi, and the quartermaster are, as soon as you pass the waterfalls, a breath bar is popping up like you’re under water. I didn’t notice it when I first went in, but I was looking at one of the merchants when I died. I was very curious as to how and the recap says that I drowned. As soon as I respawned, the breath bar came up again. If you stand next to the waterfalls at the entrance they go away, but once you pass that you can drown by just being there.
Technically, you are under water. Since the cave is under the bed of the stream that feeds that waterfall.
But the cave having an open mouth and being full of breathable air should count for something.
People are reporting that the Valdrakken bank, which is under a pond, is also drowning customers in clear air.