Police began searching a duck pond in Holywell near Castle Bytham, Lincolnshire, after the woman's car was found parked in the area Thursday night, The Evening Telegraph of Peterborough reported. The 6-year-old boy's mother had alerted police a few hours earlier that his grandmother had failed to return him after an overnight visit.
"At the moment we believe this is a tragic accident," Detective Chief Inspector Dave Wood said. "Initial inquiries show there were no witnesses, so it's not possible to say how this has occurred."
The woman's body was discovered late Thursday and the boy's at about 4 a.m. Friday, Wood said.
The boy lived with his mother in neighboring Leicestershire, while his grandmother lived in Castle Bytham. The bodies were not expected to be formally identified until Monday.