Within San Miguel, walking covers most of what you need: the malecón, the central plaza, the dive shops, and most restaurants are within a 10-minute walk. Bicycles and electric scooters can be rented cheaply.
To explore the rest of the island — Punta Sur, the east coast, the southern beaches — rent a car for the day (US$40–60), a Jeep (slightly more), or a scooter (US$25–35). The roads are well-paved and traffic is light outside town. A car is much safer than a scooter on the cross-island road, where wind and patches of sand can be hazardous.
Taxis are everywhere but expensive for longer trips: agree the price before getting in. There is no Uber on the island. Buses do not really exist in the form most travellers recognise.