My '41 Taylorcraft lived outdoors for most of the time I owned it, and I was always draining water. Fortunately, though the tank itself had no drain it had an old style glass gascolator so I could see just what was in it, and the pickup was at the low point of the tank. Only once did I have a problem, which put me on the ground in a little field out in the NJ Pine Barrens* wherupon I drained several gascolator's worth of water and crud before restarting and taking off again. I had just filled up for the first time from the FBO's brand new fuel truck and even though I sumped my tank afterwards, it must've not all settled yet.
* Once on the ground, I noticed an old windsock frame rusting in the weeds, and somebody told me later that there was an old cropduster's field in the area.
My Hatz has a single wing tank with pickups at each of the four corners, all teeing together above the gascolator. It lives indoors and I've never seen any water.