I'd be interested to know how other approach this. My shot count per dive have ranged from 0 (bad viz in a boring place) to 200 (the f*ing crazy whitetips in Cocos on a night dive -- kudos to the charge capacity of the Ike DS125).
For me, there are five levels of culling.
- As I go in the water and get to the bottom, I test fire a few shots to make sure my imitial assumptions (something like f8 and 1/250 and 1/2 power on the stobes with diffuser with a base ISO of 200) aren't too bad. I trash all of these shots, as I go.
- As I find the range, I tune the exposure -- mainly flash power and shutter speed. I blow away obvious trash as I go, if I have time.
- Immersed in a shot (say, of a goby on a coral head), I chimp and try to get the shot right. The action is too fast to bother with deleting photos. I just adjust and keep the misexposed shots.
- When I surface and review, then a good number of the shots are clearly inferior to others -- just based on different histograms of similar subjects. I blow the crap away.
- Each day, get rid rid of the obvious junk before backing up to laptop or more compact (in my case, Wolverine) device.
My 2 cents,