One of the really great items that we've been using aboard Raptor Dance since we set off on our cruise is a gadget from a company called "Soda Club" (later renamed "Soda Stream")
This unit is about the size of coffee maker and makes really tasty soft drinks (see picture below). One CO2 canister holds enough gas to carbonate about 110 liters of soda. You make up a liter batch of seltzer at a time and then add the flavoring of your choice.
They sell the flavors at their website, you can mix your own or obtain syrup elsewhere.
It's a really cost effective way to make soda, mixers and saves enormously on storage space.
We bought from the 6 extra carbonators so we don't run out as they can't ship the gas bottles to Mexico. We also stocked up on their Diet Cola, Diet Tonic, Diet Ginger Ale and Diet Lemon/Lime Soda. Their Tonic is much tastier than any we've found in Mexico.
To check out their products, click the link/graphic below (note they do give us a small referral fee if you buy their goodies after following our link).
Also, until October 15th they're running some specials for online orders:
Save $30 on any new soda maker package when you use promotion code FIZZYFALL and
Use promotion code CHEAPGAS to save $15 off any 110-liter spare, licensed carbonators.