In the past, they have had diet Sierra Mist in cans, but those are difficult to come by. So difficult, in fact, that we had all of them in our DOS on the Dawn a few years ago. Halfway through the week my husband would ask for one at a bar and they didn't have any. We ended up bringing the ones from our cabin.
There is a possible solution here, but it is not the simplest thing to do. You can call the special needs department and let them know you need sugar free ginger ale because you are a diabetic (which is true for me) that gets seasick (also true). If they grant you a liquid exemption you are able to carry on what you'd like. Of course, this is once you fight security about it. 😁
We did this for several cruises. It's not ideal, but it is something to consider.