Assuming you have access to Kwajalein Island, getting around is simple. The island is only about 3.5 kilometres long and very narrow; walking and cycling cover it comfortably. The base provides bicycles, and the flat terrain makes this the natural way to get around. There are no rental cars.
A small ferry connects Kwajalein to Ebeye Island, making the crossing multiple times a day. The trip takes around 20 minutes.
Beyond these two main islands, the rest of the atoll's 97 islands are only reachable by boat. The lagoon is large enough that travelling between the outer islands requires meaningful boat journeys. Boat access for base residents is available through the base marina; for anyone else, it is a moot point.