The 16-year-old was enjoying a fishing trip with friends when he accidentally lost the device over the side of the boat.
After the angling club refused to let him use his diving suit to retrieve the device, the boy came up with the bizarre idea of draining the pond.
The teen sneaked back later in the night with a powerful pump and two hoses, 'mirror.Co.Uk' reported.
"I thought two pumps would drain enough of the water from the pond so I could find my cellphone," he told a local paper in Cologne.
Also Read
The boy tried to direct the water into the angling club toilet but failed to notice that the toilet wasn't attached to a sewage system.
When the owner arrived to a flooded car park he quickly found the cause and called police.
The boy was ordered to pay for the damage to the toilet, the clean-up operation and the water to refill the pond.