What it does
"Water Always Wins" isn't an official Magic: The Gathering keyword or ability. It doesn't appear on any card and has no defined rules meaning in the comprehensive rules or any Commander format.
You may be thinking of a real mechanic with a similar flavor—perhaps something water or flood themed, or a custom/joke card from outside official Magic. If you can share the exact card name or where you saw this phrase, I can identify what's actually going on and explain how it works in Commander.
If you're designing a custom card, I'd be glad to help you word a "Water Always Wins" ability cleanly so it functions within Magic's rules templating and plays well in a multiplayer EDH environment.
Iconic cards with Water Always Wins
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