Our first cruise was on royal carribbean - majersty of seas. We had a very nice experience. The common compliant is the long wait on the elevators but that might be the same for carnival. We will be going on our 2nd cruise which is carnival in july. The staff were all great.
If you do book any excusion just keep in mind that it will take up most of your time. We choose the dolphin swim in naussau and it stated it was approx 3 or 4 hr cannot remember but by the time we got to the ship it was just in time to go back on ship. The good thing is if you book from the ship and they are late they will wait.
I highly recommend the dolphin. However it is very expensive but worrh it.