Police have sent the body of Samiullah Fahim, 18, to Dhaka Medical College Hospital (DMCH) for post-mortem.
Ashulia Police Station OC Mohsinul Kadir told bdnews24.com Fahim, a grade 11 student of Bangladesh Air Force Shaheen College, Dhaka, went to the Fantasy Kingdom with six to seven of his friends on Monday afternoon.
His friends found him senseless in the water while they were bathing in the Water World.
The park authorities rushed him to the local Women and Child Hospital where doctors declared him dead.
Doctor Suranjit Saha said the victim had died before he was brought to the hospital.
He could not say what caused the death.
bdnews24.com tried to reach Fantasy Kingdom authorities but no one was there.
The OC said the actual reason of the death would be known after getting the post-mortem report.