As posted by Jenny, there is a shuttle from the onsite hotels, but it leaves late, taking away the early morning advantage.
Best advice is to rent a car for your visit, even if it costs more. The convience factor makes up for the differance.
It's nice to visit SWF in the morning, and then another time at night.
What I try to tell everyone is to visit a park early in the morning, go back to your hotel and rest, then visit another park at night.