Bee stings kill more people than do sharks - but shark attacks make better movies. (Just how cool would Leonardo have looked if he'd been fighting off a bumblebee in The Beach?)
In reality, there are no aquatic animals that routinely attack humans, unless you provoke them by poking them, sticking your hand into their home, or getting too close to one with territorial instincts.
As part of your certification lessons, you'll learn some common-sense precautions. Most of these, however, are more for the protection of sea creatures than of you. You pose a much larger threat to them than they do to you. (Compare, for example, the number of humans who will eat squid sushi today, versus the number of squid who munch on human sushi - ever!)