There are tons of videos of shark attacks of animals on the surface and none of them look like that (that I've seen). Shark swims forward... bites... there would be thrashing, water churning, etc..
Most likely... he turned to swim towards the back of the boat where there was a ladder. He couldn't get there and a strong current pulled him off into the darkness before he could be rescued.
Finally, an objective explanation of an impulsive dare devil action by a young man turning tragic.
In addition to your impressions regarding shark attacks not occurring in a neat fashion, a documentary that I saw stated the following about shark attacks on humans:
Sharks are not used to humans as prey. As a result, they do not usually attack a human immediately. Rather, they will approach the human, then bump him or her one or more times with out biting. The bumps (smell) are used by the shark to:
A. Determine, well.... whether or not the unknown something tastes good (awful, but makes sense).
B. Determine whether or not the unknown something is capable of fighting back to the extent the shark could get hurt if it makes a full on attack. Predatory animals cant afford to get injured.
Like, you I strongly suspect that the poor victim was swept out by a current and drowned. There is a natural impulse to fight against currents that leads to the victim becoming exhausted and drowning. Likewise, he may of gotten disoriented and swam in the wrong direction.
I have swam in the ocean off an unlit beach following sunset. It was beautiful, especially with the phosphorecense. But.... once the sun sets completely, you can't exactly stand up to get a good view of where the beach is. Fortunately, I could see and hear the waves breaking on the beach. But.... if I had been further out?
