>The work of submerging air at atmospheric pressure vastly exceeds the potential energy value extractable from the oxygen in terms of ATP. He's using it to float.
he is wearing some armor, definitely head helmet - that allows to balance the floatation, and would be beyond most human capabilities without the goatskin. So he achieves 2 goals simultaneously - extra air for prolonged diving as well as diving while donning some armor. That goatskin is like 20-30 liters - good for several minutes of breathing and those 20-30kg of extra floatation allow for the typical armor/chainmail and weapons.
There is no issues with holding it submerged like other commenters suggest as he keeps it under the body, so the main force of those 20-30kg pushes onto his body up, and if anything it is very easy to belt it to the body so it wouldn't need any hands involvement to be maintained under the body.
There is also no issues with drawing breath as suggested by others as the goatskin is under the same water pressure as his lungs.
If you think there are no issues then try it yourself with an inflatable beach ball and some weights in the shallow end of your local swimming pool. Be sure to record a video: it's going to be hilarious watching you flail around underwater while making no forward progress.
he is wearing some armor, definitely head helmet - that allows to balance the floatation, and would be beyond most human capabilities without the goatskin. So he achieves 2 goals simultaneously - extra air for prolonged diving as well as diving while donning some armor. That goatskin is like 20-30 liters - good for several minutes of breathing and those 20-30kg of extra floatation allow for the typical armor/chainmail and weapons.
There is no issues with holding it submerged like other commenters suggest as he keeps it under the body, so the main force of those 20-30kg pushes onto his body up, and if anything it is very easy to belt it to the body so it wouldn't need any hands involvement to be maintained under the body.
There is also no issues with drawing breath as suggested by others as the goatskin is under the same water pressure as his lungs.