Department of Public Security official Elmer Román told Express that the grandfather and his granddaughter were playing a game at the time, according to Express.
They were on the 11th deck, Sky News reported.
Police are investigating and are reviewing security footage and interviewing passengers, The Sun reported.
The girl was traveling with her parents, younger brother and grandparents, Primera Hora reported.
A spokesperson for Royal Caribbean said, "We are deeply saddened by yesterday's tragic incident, and our hearts go out to the family. We've made our Care Team available to assist the family with any resources they need. Out of respect for their privacy, we do not plan to comment further on the incident," Huffington Post reported.