This was a big topic of discussion on the rotary-aviation list recently.

100LL does not carry water.  That is why you can drain water out of your tank.  But it can take a few hours for the water to settle to the bottom of the tank, so the fact that you drained it doesn' t mean the amount of water in the tank is zero (and if you didn't drain it then all bets off).  And it's theoretically possible to get some water in places where the drain won't get it out, like in between the tank and the engine.  Bends in the fuel system can trap water, like bends in the plumbing under the sink can trap hair.