Hot GPUs draw more electric power. Therefore, watercooled GPU will always draw less power than the same GPU cooled with air. Simply because watercooled GPU will always run about 30-35 °C cooler. And for every °C that the card runs hotter it needs approximately 1.2W more power to handle the exact same load.

More details here:
https://www.techpowerup.com/reviews/Zotac/GeForce_GTX_480_Amp_Edition/27.html