Also want to point out that
DCL allows you to bring your own non-alcoholic beverages (like sodas, bottled water, etc). Unlike the rules regarding beer or wine, the amount seems unlimited provided you can carry it through check-in and onto the ship (instead of leaving it with the porters).
When we sailed a 13-night itinerary, we filled a carry-on with a couple 12-packs of Diet Dr. Pepper and Sprite Zero so that my mother, who can't drink soda with sugar, could have more options. We still would go to the top deck for Coke Zero but it was nice having an alternative.
Even so, we did purchase a few non-alcoholic drinks, and while prices may have changed since 2023, we paid about $2 for a can of soda from the bars/movie kiosk, and it was around $8-9 for specialty non-alcoholic cocktails/smoothies.
There are also non-alcoholic drinks of the day that are around $6. We don't drink coffee so didn't order anything from Cove Cafe, but I believe the prices were similar in the the $5-8 range.
If you are a coffee drinker and don't care for the industrial taste of the free coffee, there are hot water stations on the top deck, or you can order a carafe of hot water from room service, and bring some packets of your preferred coffee (or tea!) to mix yourself. We brought our own tea bags and a box of oat milk (can't do dairy) for the morning pick-me-up.