The Coroner is charged with the responsibility to determine the cause of death. If he had a Cardiac event, and died in the water from that, then it can be called accidental. If the drowning was because of the swimmers inability to surface and survive in the water, it would be accidental drowning. If someone held him under the water, or prevented the swimmer from getting enough air to breath, it is a Homicide.
It could be a common practice during SEAL training to find that line where a candidate simply can't do any more. The trouble with water training at this level, is that every breath is precious to the trainee, and it may beyond the trainee's ability to have enough air to say "Stop", or "I quit". That puts the very lives of the trainees in the hands of their trainers. There could be room yet, for a finding of accidental-v-homicide. For the Coroner to call this a homicide, the Coroner feels he has enough evidence from the findings at autopsy, and subsequent interviews, to make the call of Homicide. If there were people there who heard the trainee say," Stop", or "I Quit", and the trainers ignored the cry; that would weigh heavily towards a Homicide finding. The Coroner has to make the call with a blind eye, if you will, for what is the "usual practice" during training. The same standard would apply to all drowning cases, be it SEAL training, a waterman plying his trade, or anyone in the water for recreational reasons. If I held someone under the water long enough for that person to drown, I would be guilty of Homicide by drowning.
My $.02.