I can confirm diet sprite is in the drink stations (not all of them, but most of them). I was told that you can get it in the dining room, but there's a charge because they only have it in a can (DS asked for it at our embarkation). You can also brew your own iced tea if you like it that way. There's hot water and tea bags up there, and the dining rooms have it too.
Another sugar free dining room option is club soda. I had club soda with lime every night for dinner. DH asked for tonic water and lime our first night, and was told the same thing about a charge because it was in a can, so he just had diet coke. But when we ate at Palo, he ordered tonic and lime and our server brought it. I assumed we'd be charged, but it wasn't on our bill. I'm not sure if there's sugar in tonic water or not but I know soda water is sugar free.