shallow water practice is the best way to gain capacity and confidence, or some sort of winhoff method....sitting on the couch or whatever as it is a simmilar flex that you can drop into any sort of free STATIONARY time, ie not while driving, or on top of anything like that, or before doing a dive, even just blowing your head up, blowing baloons, again within limits, as I believe that there are possible ways to injur yourself doing that, but as an easy to do (very rough) capacity test that can be repeated as often as desired for almost no cost. I got to the point where I could swim a full lap(two lengths)in an olympic pool underwater with no trouble, and only a few extra breaths before going, it seemed to be mostly up to following a very moderate and steady stroke.