The boy -- who has been named as Qwentyn Hunter -- was on a four-day Caribbean cruise on Carnival Victory with other members of his family, Carnival confirmed.
A Carnival spokeswoman said: "Parental supervision is required for children under 13. In this case there was a parent present at the time of the incident. Carnival extends its heartfelt sympathy to the family during this very difficult time. The company's CareTeam is providing assistance and support.”
The ship arrived back in Miami this morning and was boarded by Miami-Dade police who carried out an investigation, including speaking to witnesses.
"The boy was apparently playing with his 10-year-old brother in the pool when they suddenly found him at the bottom," said Detective Javier Baez of the Miami-Dade Police Department. They attempted to administer CPR however, unfortunately, they were not successful," he added.
Cruise Critic member Charleyann wrote on the Cruise Critic Message Boards that her friend had been on the ship when the incident happened: “DJ Alex spotted something floating in the pool. A passenger dove in and got child out. Paramedics, crew and one passenger worked on this child for 30 minutes as passengers looked on in horror.”
--by Adam Coulter, U.K. Editor