AirLeft: This is the amount of 'breath' that the player has left. It is a count-down timer ONLY when
1) underwater and scuba off (sector.lotag == 2 and player.scuba_on == 0)
2) player runs out of scuba amount
If it reaches zero, then Player takes damage (extra_extra8 is incremented by 32) It is initialized to 390 (15 seconds @ 26 tics/second ) when a map is loaded.