EVGA doesn't void warranty if you install a waterblock, as long as you register the card within 30 days of purchase. Even if damaged DUE to watercooling. Trust me, I've done it.
And most cards don't NEED watercooling, but they need cooling of some sort, which is why they come with fans installed. If this is a discussion of 'need' vs. 'is it beneficial?', then that should be determined.