They have the fountain drink pass. It is $6 a day for 7 days plus 15% gratuity. With it you get unlimited fountain soda drinks. They put something on your sea pass to indicate that you have purchased this package. Many people bring insulated travel mugs to fill since the glasses give smaller servings. I believe the package comes with a Coca-Cola tumbler.
From their website:
"Drink packages are an economical and convenient way to purchase select drinks at our bars and restaurants. Kids can enjoy unlimited soda, and for adults, there's a soda package and a wine and dine package that includes a bottle of wine each evening at dinner.
To purchase a drink package, visit any one of the onboard bars and lounges and the package will be charged to your SeaPass® account. After you've purchased a drink package, a sticker will be placed on your SeaPass Card to show which package you've selected. Then all you have to do is show your SeaPass Card as you normally would when ordering a drink."