f/8.

I'd be more concerned about shutter speed. If the drownee is flailing their arms, dragging the shutter slower might yield a more dramatic look with the motion blur. Too high of shutter speed so the action is frozen might look more like someone rehearsing their part in a synchronized routine.

Keeping the subject in focus is more important as a decent telephoto will get you an out of focus background. Unless you're just a dick and use a wide angle but wade out into the water to get a decent framing.