The health bar and stamina bar should be moved somewhere a tad more.. visible. Maybe the middle of the screen?
The AIR bar needs to be smack dabbed in the middle. Think I died 5 out of the 9 deaths yesterday via drowning. You could even make the air bar bubbly instead of just a simple rectangle.
Bold move to put HUD info in the middle of the screen. Perhaps along the vertical edge of the screen would be more visible?
The air gauge could flash when your air supply is running low. Sound cues are always great to alert to dangerous air levels as well. There are plenty of ways to keep the player alert without needing to resort to such drastic measures!
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