When you dive with someone as their buddy, you _do_ create a legal relationship (albeit a slight one). It is the same relationship as exists between a rescuer and a victim - because you agree to be responsible for the other diver, even if only in a small way, you keep other people from assuming that obligation.
Anyhow, here is some contrary authority (looking at several cases and distinguishing each by finding that they do not find a special duty): Barrett v. Ambient Pressure Diving, Case no. 04-CV-03550 at 22, n.40 (E.D.Pa. 2006). The cases distinguished are Kuntz v. Windjammer "Barefoot" Cruises, Ltd., 573 F.Supp. 1277, 1282
(W.D.Pa. 1983) (finding a duty to setup buddies, but not finding a special duty between buddies); Lyon v. The Ranger III, 858 F.2d 22, 24-25 (1st Cir. 1988) (finding a duty to develop a reasonable and not "seriously flawed" dive plan, but not a special duty between buddies).
I throw those out to play devil's advocate. The fact is the general duty to behave reasonably exists, whether above water or below it. The fact that some courts have not found a special duty between dive buddies does not mean that there is no duty, it means that the duty of care is no different than the duty you might have to someone you meet for drinks, or any other casual relationship. If the person starts choking on their tongue, a reasonable person might be expected to call for help. If they die and you failed to call for help, you may be on the hook. Don't do that - behave reasonably. Also, get enough indemnification that your insuror feels highly motivated to defend you.
Incidentally, on the question of what is "reasonable" - who knows? By amazing coincidence, every jury ever seated in a negligence action has discovered that _they_ are reasonable people, so what they would do in the situation is definitionally what a reasonable person would do. They almost never come back answering, "we can't tell what a reasonble person would do here."