What convinces you it's a working system? Making your enemies, and even your own people, think that you 'can even reach those in fortresses beyond a moat, and e

What convinces you it's a working system? Making your enemies, and even your own people, think that you 'can even reach those in fortresses beyond a moat, and even archers on watch won't see you coming' surely has a lot of value. If you used it as a re-breather then surely the work needed is no greater than the work needed to submerge a lungful of air? I expect you're right that it's a float (appropriate username is appropriate!) - but the balance point seems likely to me to be a small additional volume of air as well as the lungs full capacity. Otherwise maximum horizontal distance would be attained by exhaling early when swimming underwater - that's contrary to my experience - and large lung capacity would be detrimental to swimming underwater.