+1 on Nitrox, probably one of the most useful cards to have.
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I can find my way from point-to-point underwater using a compass for bearing but I can't gauge distance with pinpoint accuracy.
Just to be clear, I am not interested in card collecting. "I love to dive and I get a sense of enjoyment from 'diving well.'" I find that instruction in courses has (so far) been the best way to learn new skills while diving maintains those skills. Those skills help me to increase my enjoyment of diving.