You should be ok bringing soda on. I have carried a six pack of coke. I normally buy some cans just prior to getting back on ship. Ususally from a local store or market; then I carry it on with my bag.
Freedom of the Seas Dec 11
Allure of the Seas: Jan 11
Voyager of the Seas: May 09
Splendour of the Seas: May 07
Explorer of the Seas: Jan 04
Majesty of the Seas Jan 99
I brought a few of my own cans on RCL and Holland at nearly every port because I like particular pop brands -- only Princess staff ever stopped me and had a problem with it. They wanted people to start buying their coca-cola cards.
2003 - Princess - Glacier Bay Alaska (Dawn Princess)
2004 - Royal Caribbean - Mexican Riviera (Voyager of the Seas)
2005 - Holland America - Caribbean + Belize (Zuiderdam)