Is anyone aware of any shark attack that has taken place underwater?
Yes (apart from the fatalities that happened during feeding sessions).
In 1972, in the British Virgin Islands, one of the buddies of Bret Gilliam was killed underwater (quite deep) by two huge oceanic whitetips which repeatedly bit him and took him into the abysses. His body was never found. There had been no feeding of any kind before the accident. Sonic waves from a neighboring submarine may have excited the sharks.
There has probably been other fatalities underwater due to sharks, apart from feeding sessions (notably an Italian photographer in the Mediterranean Sea but I have no specific details).
There have also been non-fatal incidents with sharks, involving divers underwater, during baited dives. Once sharks begin to get used to divers and to associate them with food, like during baited dives or feeding sessions, it's clear that anything can happen, sooner or later, even if it's with a relatively small probability of happening - be it during the baited dive, or at another time. That's why most biologists I know condemn those practices of baiting and feeding. That's partly why I consider that baited dives, and any kind of feeding any fish, SUCK.
Size also matters - while underwater, an encounter with an oceanic whitetip shark is probably less dangerous when the shark is 1.80 meter/6 feet long than when it's 3.60 meter/12 feet long. It seems very likely that the smaller shark is less self-confident, more cautious, towards divers as big as itself.
People have been considering wolves as devils for many centuries, now some urban people consider wolves as inoffensive creatures. Truth is in-between.
Like wolves, sharks are neither angels nor devils - they are predators.