21%

...the amount of Oxygen in air.

Sorting out my dive gear today - as the plan is for a group of us to do a gentle shore dive tomorrow afternoon in Saints Bay Harbour.

I have a few tanks.

Air - 21% Oxygen & 79 Nitrogen - maximum depth would be 54.1 metres
EAN (Enriched Air Nitrox) - 32% Oxygen & 68% Nitrogen - maximum depth would be 32.8 metres

The main difference between them both is reduced body (soft and hard tissues) absorption of nitrogen with EAN and a longer dive time - but reduced depth for the dive.
"Pony" - known as a back up air tank - a smaller tank of just 3 litres.

The orange tube with my name on it is called a "Delayed Surface Marker Buoy" - which must be used when surfacing from a dive or in a Harbour to show that a diver is below.