I swim a lot - not just paddling up and down the pool but I go to proper training sessions where a nice lady shouts at me a lot (in an encouraging sort of way!) and makes me swim a lot further and a lot faster that I really want to.
Every so often I have to stop going for a while and the weight creeps back on but then, when I start going regularly again, the weight disappears. I now only get chance to go and be yelled at once a week and, while I am a lot heavier than I want to be I am not getting any bigger.
So, it is either the swimming or the fact that I am water soluble and could actually get the same effect by having regular relaxing hot baths instead of manic quick showers.