Soda Drinks (from a soda fountain) are included, at meal times, in the table service restaurants. They are not included outside of those times.
DCL has a refillable mug program. You purchase a mug for one price at the beginning of the cruise and you may get free refills at any of the lounges or fast food locations, like Pluto's or the Pizzeria.
If you want a soda drink that comes from a can, you'll have to pay for it, at $1.50 plus a mandatory 15% gratuity. An example of one that only comes in a can is a Diet Sprite.
The refillable mugs cost $5 per day, so a 7 day would be $35, a 4 day would be $20, etc. It costs the same if you buy the mug on the first day or the last day.
Alcoholic drinks are always at your expense.
They also have a free drink station on deck 9 aft that has coffee, tea, hot chocolate, and filtered water available 24 hours a day.