In the spirit of the 21st century, here is my "anecdotal evidence." I am a 59 year old male, 5'10, 200 lbs. Walking on a treadmill with no incline at a pace of 3.5 mile per hour I burn 7.5 calories per minute. On a cross country skiing trainer, moving arms and legs, at a brisk pace, I burn 15 to 16 calories per minute. Leaving aside individual metabolic difference, body type and age, I can tell you that the times in diving when I am expending more than the calorie burn rate of a brisk walk are when surface swimming in full gear, when climbing onto a boat or pier in full gear, and perhaps donning and removing my or my biddies gear. That's about it. However, being in water colder than body temperature increases the calorie burn , as the body restores lost heat. Diving in 80 degree water off a boat on a calm day, with limited surface swims, I therefor accept the 8 calories a minute generalization presented above, maybe a little more when in colder water.
When training with students in a local 60 degree reservoir, with a lot more movement that I normally have on a recreational dive as we do search and recovery and navigation problems, with lots of surface swims out and back from training platforms, and repeated donning and removal of gear during the day, that number goes up, maybe to 12 calories a minute- about where jogging at a 10 minute mile pace puts me. Scuba to me is at its best when it is like underwater meditation. If I need to burn calories, I'll load and offload tanks on and from the truck or van.
DivemasterDennis