They also have the Red Souvenir Bottles available for $15.99, however after the first day of free drinks, they charge $0.99 per refill or $5.99 for one day of unlimited refills.
Possible similar deals at other Six Flags Parks. YMMV.
Click on "Refreshing Drinks" Buy Now Button
You should see the Green Souvenir Bottle Voucher.
Don't forget to use your Discover Card for 5% cash back.