Most dive lodges will lend you a bicycle, which is the way most people get around — the island is flat, the single paved loop road is 25 kilometres around, and the speed limit is 25 mph specifically to protect the iguanas. There are no traffic lights, almost no other vehicles, and you can cycle from end to end of the developed part of the island in under an hour.
A few rental cars are available from the lodges or the small operation at the airport; if you intend to drive to Point of Sand at the eastern tip or want to explore independently, a car helps. There are no taxis to speak of — the same person who picked you up at the airport will usually take you back to it.