Are buying shore excursions ahead really required or we can buy when we get down the ship in nassaus,baham ?

Question

Are buying shore excursions ahead really required or we can but when we get down the cruise ship or we can just go by taxi.We are going on aug 31st 2007 on carnival cruise, so some body please help us to find best shore excurions and how to book them.or if we dont need to buy ahead and if its possible to buy on shore we want to do that please some one help us