It’s the surface that’s the dangerous spot. You’re at the boundry of two worlds. I thing I’ve noticed during my few dives (I’ve only done my certification dives) and been told also by veteran divers is that once you are under the water most of the critters really don’t care about you since you’re pretty much part of their world. It’s when you’re on the surface that you’re seen as prey (since animals like sharks like to ambush from beneath).
Yes. We scuba divers are most vulnerable at the surface or when engaged in spear fishing activities. No different than surfers. I’m not so sure about our absolute safety while submerged, however. Sharks may also be aggressive for reasons other than actual hunger. Territoriality is another possibility. I’ve seen grey reef sharks swim past casually. I’ve also seen them drop their pectoral fins & arch their backs in a threatening manner. So who knows?
One thing is for sure. There’s not a darn thing you are going to do a out it if a large shark takes a run at you. So you either adopt a realistic attitude or stay out of the ocean.