The boy is thought to have been playing with friends at the reserve at around 3pm on Tuesday.
Emergency services were called to the scene following reports of concern for the safety of the boy in the water.
Specialist officers from Humberside Police's underwater search unit conducted a thorough search of the water and made the tragic discovery.
Detective Chief Inspector Grant Taylor said: “Our thoughts are with the boy’s family and loved ones at this very sad and difficult time. They are being supported by our specialist trained officers, as they come to terms with the tragic incident.
“The local neighbourhood police team will continue to support and speak to residents in the area and I encourage people to talk to them if they have any concerns.
“As part of our investigation we are now appealing for anyone who may have witnessed the incident to please contact us.
“We’d particularly like to speak with a man who we believe may have seen what happened and hold key information that would help with our enquiries. He is described as wearing a black polo shirt with grey denim shorts, and is reported to have white hair and a beard.
“If this man was you, or you have any information that would help with our enquiries, no matter how small, please contact us on 101 quoting log 370 of 26 July 2021.
Police across the country have urged people to "exercise extreme caution" when swimming in open waters after six people – including four boys – died in a series of incidents over the weekend.
The youngest victim was a boy of nine, while a seven-year-old boy is also fighting for his life in hospital after they both got into difficulty in the water near Pulpit Rock, at Loch Lomond, south of Ardlui, Scotland, on Saturday evening.
That incident, which also claimed the lives of a 41-year-old man and a 29-year-old woman, was the fourth such incident in Scotland’s waters in just over 24 hours.
The first fatality occurred on Friday evening, when a 16-year-old boy died in the water at Balloch Country Park, at the southern end of Loch Lomond.
Then an 11-year-old boy was pronounced dead at the scene after being found in the river at Alexander Hamilton Memorial Park, in Stonehouse, South Lanarkshire, on Saturday afternoon.
The body of a 13-year-old boy was also pulled out of the water at Hazelbank, in Lanark, on Saturday evening.
Meanwhile, tributes have been paid to two men who died while swimming in open waters in Yorkshire last week.
Ben Marklew, 19, from Glusburn, near Keighley, got into difficulties at Linton Weir, near Grassington in the Yorkshire Dales, last Thursday.
David Fell was killed in a rip tide incident at Reighton Gap, near Filey, on the same day.
Members of the public managed to pull his two children to safety but Mr Fell, from Wigan, was dragged further out.
The 55-year-old was rescued by a lifeboat crew and given emergency treatment, but could not be saved.