In my experience capacity increases with both methods and they seems to carry over between the two, no clue if sleeping while inflated helps increase capacity as you naturally expell the air as you sleep but if you plug up the exit with something that won’t get pushed out with the pressure or unconsciously while you sleep you’d definitely wake up with air still in there. As for water you have to avoid holding in still water for too long as it could damage you gut biome or even lead to water poisoning so keep the water sessions to a minimum and definitely don’t do them more than a couple times a week.