Police say officers were told four-year-old Terrel Joshua Elibox was given a ride by a kite surfer on Sunday, fell into the sea and submerged.
A search and rescue mission was immediately conducted, but police said in a release that the boy's body was not recovered until Tuesday morning.
Police say a person — identified by St. Lucia News Online as a Canadian man — is in custody assisting with the investigation.
TV station News4orce reports the boy was attending an independence day church picnic at the beach with his family on Sunday.
The report says family members of the boy — known at TJ — had no idea he was missing until a man approached his grandmother and said he'd taken the boy on his surfboard and he'd fallen off.
"TJ's grandmother did not under any circumstances give... the visitor riding the surfboard permission to take TJ on the surfboard," his aunt, Callista Roserie, told the station.