A much discussed topic in the diving medicine community, we still can't say for certain, but it seems we're homing in on consensus
The old standard was 13 METs (by Dr. Fred Bove). However, several years ago some eminent diving medicine cardiologists suggested that while a diver who can attain 12-13 METS is "good to go" in any sport diving activity, those who can achieve ~60% of that (~6-7 METS), the equivalent of walking 2 miles in <25 mins, should be able to safely dive in most rec diving situations. They added that at that at level of fitness could still put divers at risk should they find themselves in the extremes of diving in terms of cold, current, rescue, etc., situations.
The latest hot-off-the-press research appears consistent with this position:
"Diving Hyperb Med
. 2014 Jun;44(2):74-8.
Exercise intensity inferred from air consumption during recreational scuba diving.
Buzzacott P1, Pollock NW2, Rosenberg M3.
Author information 1Adjunct Lecturer, School of Sport Science, Exercise and Health, the University of Western Australia, Perth, Australia, Marie Curie Research Fellow, Laboratoire Optimisation des Régulations Physiologiques, Université de Bretagne Occidentale, UFR Sciences et Techniques, 6 avenue Le Gorgeu, CS 93837, Brest Cedex 3, France, Phone: +33-(0)2-9801-6235, E-mail: firstname.lastname@example.orgResearch
Director, Divers Alert Network, Center for Hyperbaric Medicine and Environmental Physiology, Duke University Medical Center, Durham, NC, USA.3Director of the Health Promotion Evaluation Unit, School of Sport Science, Exercise and Health, the University of Western Australia, Perth, Australia.
INTRODUCTION: Episodic exercise is a risk factor for acute cardiac events and cardiac complications are increasingly recognized in fatalities during recreational scuba diving. What is not known is the exercise intensity involved in typical recreational diving.
METHODS: This study used pre- to post-dive gas cylinder pressure drop to estimate air consumption and, from that, exercise intensity during recreational dives. Dive profiles were captured electronically and divers self-reported cylinder pressure changes, perceived workload, thermal status and any problems during dives. Mean surface air consumption (SAC) rate per kg body weight and mean exercise intensity (reported in metabolic equivalents, MET multiples of assumed resting metabolic rate of 3.5 mL·kg⁻¹·min⁻¹) were then estimated. Data are reported as mean ± standard deviation.
RESULTS: A total of 959 recreational air dives (20 ± 9 meters' sea water maximum depth; 50 ± 12 min underwater time) by 139 divers (42 ± 10 y age; 11 ± 10 y of diving; 12% smokers; 73% male) were monitored. Problems were reported with 129/959 dives: buoyancy (45%), equalization (38%), rapid ascent (10%), vertigo (5%) and other (2%). Assuming a 10% overestimate due to cylinder cooling and uncontrolled gas loss, the estimated exercise intensity associated with monitored dives was 5 ± 1 MET. Mean ± 2SD, or 7 MET, captures the effort associated with the vast majority of dives monitored.
CONCLUSIONS: Our estimates suggest that uncomplicated recreational dives require moderate-intensity energy expenditure to complete, with a 7-MET capacity generally adequate. Higher levels of aerobic fitness are still strongly recommended to ensure ample reserves.
Further research is needed to quantify energetic demands of recreational diving during both typical and emergent events in both experienced and less experienced divers."