I've never been on Holland America, but I've taken several cruises. Each one had a similar policy about sodas. Depending on the cruise line, you can purchase a refillable glass or a a sticker that is placed on you key-pass Each time you want a soda, you present the glass or show the sticker on your key-pass. I think some lines has something equivalent for alcoholic beverages, but I don't drink alcohol so I'm not sure about that.
Coffee (except specilaty coffees), tea, milk and juices are free all the time and available all the time, whether you go to a dining room, buffet venue or call room service. Some cruise lines even have a supply of tea and instant coffee with sugar and powder creamers in your stateroom,and that supply is refereshed daily. There may be a charge for certain juices at dinner - ask your waiter or barperson when you order.
Remember, that for each beverage you purchase (including alcohol, specialty coffees and sodas) you will have to pay a service charge. The service charge may vary from cruise line to cruise line, and since I don't drink much of any of those, I couldn't tell you exactly what the service charge is. On the lat cruise I took, service charges (ie tips) were charged at 18%.
Usually free drinks include ice tea, coffee, and lemonade. You can order with dinner, or get any time at stations in the buffet area. Some cruises also include free juices at the juice bars.