no city guagua goes to sosua. the connection is via POP - san juan guaguas. they pass sosua and in POP they go to parque central (or used to, not so sure now). guaguas C and F start (or end, depends how you look at it) in cofresi, on the main road. then C goes along manolo tavarez justo and fallow main street to entrada de maimon. so change your long distance guagua in the area between playa dorada and the hospital.
guagua F turns off manolo tavarez justo and drives though barrios. not sure where it terminates after the hospital thou.
the ride, regardless of the distance is 25 pesos.