#1 Complacency
1+ for complacency referring to non-combatant military and commercial oilfield divers. The work can get very routine, especially on the support side. Ill-trained Scuba divers playing at commercial in fisheries, harbors, and rivers are another matter.
The vast majority of accidents are industrial in nature, usually having to do with getting body parts between a load and target. There are a lot of commercial divers missing digits. There are very few pure diving accidents anymore.