Dolly, it's a piece of cake to get to Boca Chica. Take a taxi from anywhere in Santo Domingo, and just tell them to take you to Parque de Independencia and that you are looking for the guagua to go to Boca Chica.
Parque Independencia is the square where Duarte is buried, in the Zona Colonial, right at the west end of El Conde (the pedestrian mall in Zona Colonial). The guaguas are on the street along the north side of this square.
The guagua will go right down into the heart of Boca Chica , near the beach and all of the restaurants, etc on Calle Duarte in boca Chica. You can get a bus back to Santo Domingo in the afternoon easily too.
And ALSO. you can take your pick of guaguas. If the one that pulls up is old and full of termites, or has a full load of smelly-looking people, pass on it and wait for the next one that meets your approval. IMHO , most guaguas are in better condition than most taxis, and the guagua drivers are more polite. They just shut up and drive.