St Lucia presently only has one Cruise dock and it is in the City of Castries. The St Lucia cruise dock is also known as the Castries harbor.
On your way out of the cruise dock, right next to the immigration department, there is a shopping mall called La Place carenage where you can also find a car rental company if you want to rent a vehicle for a day.
If you dock on the opposite side of La Place Carenage, you will end up at duty free Pointe Seraphine and you will find a car rental company there as well to rent a vehicle that suites you.