Is ti better to book cruise excursions before sailing or when on the ship?

Well I agree with Mary. If its an excursion you really have your heart set on taking, you'd best book it in advance. The excursion prices don't go down once you set sail. However, if its a popular excursion, if you don't book in advance, you may miss the opportunity once you set sail. Excursions can sale out. Best be sure! Better safe than sorry. Book in advance. If you change your mind once you set sail, you can always get a credit.