There is no direct bus to Maruata. Take any second-class bus running along Highway 200 toward Lázaro Cárdenas and tell the driver you want to get off at Maruata. The drop-off point is on the highway; from there it is about one kilometre through the village to reach the beach. Buses running in both directions along this coast pass through several times a day, but schedules are loose — ask locally once you arrive about departure times for when you want to leave.
The nearest large town is Lázaro Cárdenas, about 90 kilometres south, which has a bus terminal with connections to Manzanillo, Colima, and Mexico City.