Weather wise, you couldn't have picked a better time of the year. March temperatures in the Dominican Republic range from an average low of 71.0 degrees to an average high of 83.0 degrees... little rain except for an occasional afternoon or early evening thunderstorm.
The only down side would be that it's getting perilously close to Spring Break for the Norte Americano college students, who tend to flood the islands and the east coast of Mexico. However the peak of the phemoena is a round Easter.
Best of travels!