Emergency services were called at around 3.55pm on Saturday to reports of the child getting into difficulty at Liquid Leisure near Datchet.
She was found at around 5.10pm and rushed to Wexham Park Hospital but died, the force said.
Local policing area commander for Windsor and Maidenhead, Superintendent Michael Greenwood, said: “My thoughts are with the family and friends of the girl who has died as a result of this tragic incident.
“We are in the early stages of investigating this incident to understand the full circumstances.
“There was a swift response from all emergency services and following an extensive search of the lake, the girl, who was 11, was located at around 5.10pm and taken to hospital but sadly died.
“This has been an extremely traumatic and upsetting incident for all involved.
“I am aware that several members of the public entered the lake shortly after the girl got into difficultly, but were unable to locate her.
“I would like to commend them for their courage and bravery.
“My sincere condolences are with the girl’s family and friends, and I would ask that their privacy is respected at this incredibly traumatic and distressing time for them.”
This has been an extremely traumatic and upsetting incident for all involved
One eyewitness told Sky News the child had been seen going underwater and “didn’t come back up”, at which point lifeguards rushed over.
She had been with a group of friends celebrating another girl’s birthday when she disappeared, they said.
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 at the scene on Saturday afternoon.
Members of the public were asked to call the girl’s name and staff asked customers for goggles to search for her underwater, according to witnesses quoted by Sky.
“The lifeguards were working incredibly hard,” the first witness said.
“After about 30 minutes, we were asked to leave the beach, just as the ambulance and fire crew arrived at the site. There was a police helicopter, air ambulance, and a boat for search and rescue.
“I saw one of the adults from the group which the girl had been with, and I asked if I could do anything to help, and she just asked that I prayed for her.”
The girl’s death is being treated as unexplained and an investigation has been launched, Thames Valley Police said.
Although formal identification has yet to take place, the girl’s next of kin have been informed and they are being offered support by officers.
Liquid Leisure describes itself as home to Europe’s largest aqua park and offers activities including wakeboarding and a floating obstacle course, according to its website.
“Liquid Leisure is home to Europe’s Largest Aqua Park and the first official Ninja Warrior UK Aqua Park, set on one of the most beautiful private freshwater lakes amongst stunning parkland in the heart of Berkshire,” its website reads.
Liquid Leisure, South Central Ambulance Service and Royal Berkshire Fire and Rescue have been contacted for comment.