While it won’t “ruin” granite, it can cause discoloration. Hard water, especially, can leave behind white streaks or residue from the mineral contents. That said, water doesn’t so much damage the stone or sealant on the granite as it does cause staining. When the stone absorbs water, it will often cause discoloration to the stone.

Generally, the best practice is not to allow liquid (including water) to sit on the countertops longer than necessary.