I usually carry on two or four 2-litres or a 24 pack (give the 24 to the porters. They'll get it to your room). I abhor the taste of Diet Coke (I do like Coke, but on a cruise I'd rather use the calories on dinner -- though I probably eat 15 cokes' worth of food), so Diet Pepsi from shore is the only way I'll drink it.
Do any cruise lines use Pepsi products? I've only been on Celebrity, RCCL, and Princess.