At around 3:55 p.m. Saturday, emergency services were called after reports that the boy was having trouble at Liquid Leisure near Datchet.
Power was found around 5:10 p.m. and taken to Wexham Park Hospital, but died.
Inspector Michael Greenwood, the local police district commander for Windsor and Maidenhead, said: “My thoughts are with the family and friends of the girl who died as a result of this tragic event.
“We are in the early stages of investigating this incident to understand all the circumstances.
“After a quick response from all emergency services and an extensive search of the lake, the 11-year-old girl was found around 5:10pm and was hospitalized but sadly died.
“This has been an extremely traumatic and upsetting event for everyone involved.
“I am aware that a few members of the public entered the lake shortly after the girl’s forced entry, but they could not find her.
“I want to commend them for their courage and courage.
“I offer my deepest condolences to the girl’s family and friends and ask that their privacy be respected at this incredibly traumatic and distressing time for them.”
This has been an extremely traumatic and upsetting event for all involved.
An eyewitness told Sky News that the boy was underwater and “did not go up again”, at which point lifeboats rushed.
They said he was with a group of friends celebrating another girl’s birthday when she disappeared.
Emergency services including Royal Berkshire Fire and Rescue Service, Buckinghamshire Fire and Rescue Service, Surrey Fire and Rescue Service and South Central Ambulance Service were on the scene Saturday afternoon.
According to witnesses cited by Sky, the public was asked to name the girl, and staff asked customers for glasses to search for her underwater.
“The lifeguards were working incredibly hard,” the first witness said.
“After about 30 minutes, when the full ambulance and fire department arrived on the scene, we were asked to leave the beach. There was a police helicopter, an air ambulance and a search and rescue boat.
“I saw one of the adults in the group the girl was with and asked if I could do anything to help and she just wanted me to pray for her.”
Thames Valley Police said the girl’s death was inexplicably handled and an investigation was launched.
Although no official identification has been made yet, the relatives of the girl have been informed and are offered support by the officers.
Liquid Leisure describes itself as home to Europe’s largest water park and offers activities such as water skiing and a floating obstacle course, according to its website.
“Liquid Leisure is home to Europe’s Largest Water Park and the first official Ninja Warrior UK Aqua Park, set on one of the most beautiful private freshwater lakes amongst stunning parklands in the heart of Berkshire.”
Consultations were made with Liquid Leisure, South Central Ambulance Service and Royal Berkshire Fire and Rescue.