There are several ferries a day the run between the port of Praslin, and La Digue. costs round trip are aproximatley 40 euro as of 2011. There are also several locals of Praslin that offer boats for hire, however this option is very expensive. Expect to pay aproximatly 600 euro and upwards. I f you go this route however, you will have the chance at stopping at some of the other less excesable islands in the area as well.
If you arrive by boat, you will see many companys offering bikes for rent. these are the cheapest, and really the only way to travel around the island. Bikes are about 8 euros for a whole day, just make sure you test your bike before you take it. If you arnt going very far, there are also ox carts (a buggy pulled by an ox) that you can catch a ride on.