Congratulations, you’re a newly certified open-water diver! Now what? For many people, the next step is deciding what gear you need to buy and what you can comfortably rent as you figure out whether scuba diving will become your new passion. (It will, of course).  The short answer is that you needn’t go straight from your course to the sales counter, laden down with every conceivable piece of gear. A few key pieces are worth an early investment, however, and...