One thing to remember is that fish, being cold blooded, probably require far less oxygen for metabolism than mammals who need to sustain a steady body temp. This is supported by the fact that warm blooded marine mammals breathe from the surface; I suspect if it were practical for warm blooded animals to extract oxygen from sea water, some would have evolved to do this.

Regarding divers' air consumption, as Jose said, it's typical to use 80 cubic feet of air in 30-60 mins depending on depth. Some divers use blends of air that contain up to 40% oxygen, but this doesn't significantly reduce consumption, it just lessens the build up of nitrogen, and diving depth is strictly limited when using these blends due to O2 toxicity. So you're talking about maybe 20 cubic ft of O2/hour with lots of variables. The biggest one is that we breathe by equalizing (actually increasing and decreasing) pressure inside our lungs with the ambient pressure. This is why a diver needs several times as much air at depth than at the surface; the air in our lungs at depth has to be higher pressure to allow our ribcage to expand under the high pressure of sea water.