I can't possibly improve upon @boulderjohn's post, but I completely agree. Doing the CESA horizontally in the pool gave me exactly that mistaken impression--that it was quite difficult to cover such a distance on a single breath, and basically impossible for someone of my marginal fitness level unless I started with full lungs and exhaled sparingly while kicking hard all the way to get there in time. Doing it vertically was a completely different experience, as I felt the air expanding to pour out of my lungs, while I became so buoyant I had to work to slow my ascent almost the whole way. Even if I never need to do a CESA in real life, I feel like this exercise made me understand Boyle's law in a visceral way that stayed with me far better than any classroom lesson. It also made me far less likely to panic in an OOA situation, because I know that ascending on one breath is much easier than swimming that distance in a pool on one breath.