The Carnival Victory was on the last leg of a four-day Caribbean cruise when the boy drowned in the midship pool.
He was at the pool area with other family members at the time, the statement said.
Police said the drowning appeared to be an accident and an investigation was under way. The boy was identified as Qwentyn Hunter of Florida.
“To the best of our knowledge it is the first time a child has drowned aboard one of our ships,” Carnival spokeswoman Joyce Oliva said.
Investigators said the boy was with his 10-year-old brother in the pool at the time.
Passengers immediately pulled the boy from the water and began CPR, but the child was pronounced dead at the scene.
The ship at the Port of Miami with 3,094 guests on the ship and approximately 1,100 staff members, Carnival said.
The Miami-based cruise company has 24 ships that attract an average of 4.5 million passengers a year.