Single file is the hardest formation to manage, which is why it really isn't a good one unless you are moving along a wall or hull. If you have structure, you can sit a little bit head into the structure, where it doesn't take as much of a look to see behind you. In poor viz, you can use lights along the wall to stay in touch.
But apart from those dives, side-by-side is a much better arrangement. If you hold your hands out in front of you, you also lengthen your body, so your hands can be in your buddy's peripheral vision, even if your body is not.
What generally happens as I see it is that somebody agrees to lead the dive, and everybody else plays duckling and follows. That's a REALLY hard formation to manage.
But apart from those dives, side-by-side is a much better arrangement. If you hold your hands out in front of you, you also lengthen your body, so your hands can be in your buddy's peripheral vision, even if your body is not.
What generally happens as I see it is that somebody agrees to lead the dive, and everybody else plays duckling and follows. That's a REALLY hard formation to manage.