We have taken sodas and bottled water every time w no problem. Take them with your carry on, not check in bags unless like Razer said, tag them outside your bags. But then you have to wait for them to be delivered to your room.
We have been cruising w RCCL since 2013 and always book onboard. I have always found that we get more OBC and deposit is less. We booked on our cruise earlier this month and got $500 OBC for booking a JS. I always research before we go and keep watch after we book and other than an occasional price drop, which they will honor, I have not found any reason not to book onboard, especially if you book those first 3 days and get extra discount.