We have never had a problem with bringing our own soda. We usually bring the 20oz bottles so we can use what we need and re-cap th rest for later. You are probably safe with a couple of six packs before they would consider it excessive. As long as you are not trying to fly to the port with 50lbs of coke in a suitcase
We have also just brough a couple of 2 liter bottles.
And for water, I have not seen them restrict water. But again, be reasonable, you can always refill the water bottles if need be.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013