We have found diet cordial, or cordial at all difficult to get in the USA and Caribbean
and many countires. It was quite available in asia
and 1/4 of the price
Diet Tonic would be a difficult comodity!
And then you have your gas bottles.
One of the things about cruising is that at times we must live much more simply than we have been used to.
No diet tonic? Then drink what the health
conscious locals drink!