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I can free dive to 40' although I can't do much else once I'm down there. If I take the 3 cubic ft of air down with me, will it be enough to get me that fish?
It will be enough to get a court appearance if you're in a place that doesn't allow SCUBA. And if it does allow SCUBA, a spare are is just a swift kick in Darwin's balls.
Unless you're trying to escape from a slightly submerged, crashed helicopter, the Spare Air really has no purpose in life.
It's just enough air to run out twice on the same dive.
flots.