The yellow tank? That diver is using a rebreather, so chances are it's bailout gas, so if he floods the loop of the rebreather rendering it useless, he still has an air supply to return to the surface.

Either that or a decompression gas mixture, which is higher in O2 than air to reduce the time needed to return to the surface, but it doesn't have any visible depth markings, so might not be.

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Air is for pussies.