I know on RCCL the only diet sodas that I've seen available (either in the fountain or in the can) are Diet Coke and Diet Sprite. You can bring your own soda on either line.

I would not use a prepaid CC. I know that American Express Travel Cards are specifically disallowed from use, but I don't know which others won't work. If you don't have a regular credit card and don't want to use your debit card, then just bring cash to use as a deposit on your onboard account or get traveler's checks.
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Wrona

3/02 Sovereign of the Seas Bahamas, 5/03 Voyager of the Seas Western Caribbean, 1/05 Voyager of the Seas Eastern Caribbean, 12/05 Rhapsody of the Seas Western Caribbean, 2/06 Brilliance of the Seas Partial Panama Canal(Live posts or photos), 3/06 Serenade of the Seas Southern Caribbean (Review or photos), 3/06 Monarch of the Seas Mexico, 2/07 Jewel of the Seas Eastern Caribbean (Mustard of the Seas 1) (View photos), 1/08 Maui Trip (View photos), 5/08 Rhapsody of the Seas Alaska (trip report, photos, Compasses/Menus/Other Info, review, review with pictures), 5/09 Serenade of the Seas Alaska (View Photos), 11/09 Radiance of the Seas Mexico (View Photos), Jewel of the Seas WB TA 09/10 (Review, View Photos), 8/11 Peru & Machu Picchu (View Photos), 8/12 Colombia (View Photos), 10/12 Jewel of the Seas Canada/New England (View Photos), 2012 Christmas Markets Salzburg/Munich (View Photos), 4/13 Jewel of the Seas Cozumel (View Photos), 8/13 Ghana (View Photos-Part 1 & Part 2), 11/13 Oasis of the Seas Eastern Caribbean (Review, View Photos), 1/14 Adventure of the Seas Southern Caribbean (View Photos), 9/14 Jewel of the Seas Southern Caribbean (View Photos), 11/14 Celebrity Summit Southern Caribbean (View Photos), 5/15 Jaco Beach, Costa Rica (View Photos), 6/15 AmaSerena Danube River (View Photos), 7/15 Majesty of the Seas Bahamas (View Photos), 8/15 Navigator of the Seas Western Caribbean, 1/16 Celebrity Summit Southern Caribbean, 11/16 Celebrity Silhouette WB TA, 1/17 Celebrity Infinity South America