Well, my Mariner cruise has been over a year ago. But if I remember right they tasted ok. They gave you the drinks in a cup or glass. Not the can or plastic bottle. The only soda I drank was Diet Coke. It satisfied my craving for pop. It didn't taste like the Diet Coke from a bottle after it's been sitting in the freezer for some time, but it was satisfactory.
Another thing, I usually drink Caffeine Free Diet Coke. My sister told me the soda package only covers regular soda's, not premium soda like Caffeine Free Diet Coke. Diet Coke was included in the soda package. So all I ordered with the soda package was Diet Coke.
Again, If you are a heavy soda drinker like me then the soda package was well worth it. To go up to a bar and just flash your sea pass card and they get you a soda was nice.