On every boat dive, I take a compass heading to the closet land mass. If the boat needs to leave for an emergency, or the boat drifts away, or if a storm/fog rolls in, I want to know which way I need to start swimming to get to land.
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I think this is really good advice. I have read stories of dive couples being left behind on boats that have different dive groups. (Only a few, but I guess it only takes one to make it up close and personal.) If you get left behind, it would be comforting to know that you were at least swimming in the right direction!