Drinks such as iced tea, lemonade, fruit punch etc. are free. You get your free drinks from dispensers in the buffet area.
=secret The glasses provided for these beverages are pretty small.
You can purchase a large sports bottle in the ship’s gift shop for $3 ~ $4. Get one of those for your free drinks and fill it up. That way you'll still have something left by the time you get out on deck. It makes a nice souvenir after the cruise too. Any bartender will fill your bottle with ice water for free. It's a good idea to take advantage of that since bottled water on the ship costs between $2.50 and $5. Remember to take some water with you as you leave the ship. Water on the ship is safe but water in some ports of call could make you ill so drink water from the ship or bottled water only. In the ports ice cubes can be made from contaminated water so be careful of what you drink. When you're ashore drink refreshments from cans or bottles only and avoid ice.