Emergency services were called to reports of a boy getting into difficulty in the water at about 14:30 BST.
“With his tragic and unexpected passing on Sunday at Swanholme Lakes, we would like to raise awareness about children playing near any rivers or lakes in the hot weather,” Carl said.
“Please can all parents, friends and family make their children aware of the dangers surrounding water.”
On Monday, a statement from Declan’s school said he would be “sorely missed”.
The statement from Priory City of Lincoln Academy said: “This loss is devastating and our entire Priory Lincoln learning community is grieving alongside Declan’s family and friends.”