Nothing is guaranteed in scuba diving. We all know that the ocean delivers what it wants to deliver each time we descend. Our sport is defined by that unpredictability. Unforgettable moments arrive; we savor them; and we dive again the next day hoping for more.  The first time you saw a turtle underwater or watched a manta ray glide by; the first time a teeming coral reef came into focus or a shipwreck appeared out of the gloom—all of these...