We went on the Majesty in March, and we arrived at the port about 11:00, and they had just begun boarding. This way, you can beat the crowd, and enjoy the ship for the full day before you leave port. You can go up to the lido deck for lunch as soon as you get on board. This time would be good, as long as the ship cleared customs and all the passengers are off and they are ready to begin boarding.
Have a great cruise, we did!