Thinking on, regarding tips...
I always keep some fuel in the drainer between drain valves - the reasoning I was given by another instructor during my BCPL is that in doing so the likelyhood of getting water from all three (or however many) drain valves is highly improbable - and thus should you get just water from one it will be instantly identifyable.
The theory works - always done it - but never found any tank provide a 'water only sample' yet.
Hope this helps,. .FF