Cups are associated with the Water element because they deal with the emotions and feelings, being creative and artistic, and even intuitivness.
They get their energy from Binah, or the creative world, and reflect the water signs of the Zodiac. (Pisces, Cancer, and Scorpio)
With this suit, I think of the Moon, and it's energies. If you look at the shape and nature of a cup it's receptive-- like the Yin aspect of things.