When you check in, your luggage is usually taken to your cabin for you. This can take a couple of hours before it arrives in your cabin. Take anything you may need in the meantime with you in hand luggage.
Day at sea- always the busiest time on board, if you want a sunbed on deck, you will need to reserve one early as there are never enough!
If you are cruise only, and no flights involved, an extra holdall/ bag is useful for your duty free/ gift etc, also no luggage restrictions so you can take the extra clothes/ shoes you don't know if you will need or not!