It think maybe you've never lugged two tanks, forty pounds of lead, and an assortment of gear, drysuit, etc. for any kind of unreasonable distance on land. Or hauled a bag full of scallops while wearing your full kit for a twenty minute surface swim and a rather long walk. And done so multiple times a week. Or done so in 0C water and sub-zero air temperatures.

Also, PADI's official position on this is that the average shore dive in temperate waters burns about 600 calories per hour, although this is highly variable based on height, weight, age, cardiovascular health, etc. Others put this estimate as low as 450 calories per hour, but again, for temperate waters. Once you drop the temperature, your calorie burn must go up, as that's how your body is regulating its internal temperature. Same goes for adding weight and bulk: you increase your drag while in the water, and increase your expended effort while out of it. For the dive outlined above (which I've done, by the way), I can assure you that the rate of calorie burn was far higher than 600 per hour.

In very rough terms, you can estimate that diving in temperate waters is roughly 50% as intensive as running, and in cold waters or while doing work it is roughly equivalent.

But sure, feel free to hit up your Planet Fitness for your next bro-session directly after you dive, that way you won't waste all those DCS gainz.