I was mining underwater, and there was a lip of air (it was a stone wall, with a water pool on the floor next to it. I started mining into the wall, and so water rushed in. You know when the water isnt level anymore, so there was a lip from where the water reached the top and the next block above it.) that I went up to breathe in, so I stayed there thinking that I was above water because there was no "air bar" ( the bar that shows how much air you have underwater) I continued mining in the "pocket of air" when my hearts started going down. Before I could figure out what was going on, I drowned. If I really was underwater, why wasn't there an air bar?
If it is a bug, maybe you could fix it so that if you do go into the pocket of air, the air bar will appear so you don't drown expectantly.
I've noticed this before, but I'm almost positive this is a duplicate. What happens is that the range you are in water is different thatn the range you see the air gauge.