He was rushed to the Merit Health System in Biloxi, where he was pronounced dead just after 5pm, Harrison County Coroner Brian Switzer said.
The boy’s name has not yet been released.
Captain Houseman said a lifeguard was on duty at the time of the drowning.
Witnesses told WLOX-TV the pool area was packed with guests on Sunday as temperatures soared above 32C (90F), and described the scene as “chaotic”.
In a statement, Margaritaville Resorts told the Sun Herald they were “deeply saddened by this tragic accident”.
“We are heartbroken and our prayers are with the parents and family in this difficult time,” the statement read.
According to the resort’s website, its pool playground has a 450-foot lazy river, lounge pool, a water ropes course and slides.
The Independent has contacted Margaritaville Resorts.