I
The three leading modifiable causes of death and disabling injuries were again cardiac events, insufficient gas and arterial gas embolism. That more than half of all recreational diving fatalities were aged 50 or over highlights the importance of maintaining fitness to dive as divers age. High blood pressure and heart disease were the most common pre-existing health conditions known among 2014 diving fatalities, and obesity was more common at 51% than found in the general living population at 35%. It may not be unreasonable to speculate that older, heavier divers with pre-existing heart or blood-pressure conditions are at elevated risk of dying while scuba diving, compared with younger, healthier divers.
Also, from reading the case reports, it seems like many deaths were due to inadequate training, not being familiar with the equipment being used.
Diving at night with only one light, using a brand new BCD that you do not know how to inflate/ditch the weights, etc.