The 17-year-old's body was retrieved by police divers after almost five hours of searching. The boy is believed to be from London and had been swimming in the reservoir with friends, who raised the alarm after he got into difficulty.
David Wright, manager of West Yorkshire Police's underwater search unit, said: "At this time of year the weather is starting to get a lot warmer and people who are out enjoying themselves in the sunshine often see a reservoir or canal as a good place to cool down or play in.
"What people don't always consider is that these waterways aren't like swimming pools. They can be extremely dangerous places, even killers."
Paul Mudd, Yorkshire Ambulance Service's assistant director of operations for Calderdale, Bradford, Kirklees, said: "Water can look calm on the surface but contain unseen debris and rivers in particular can have treacherous undercurrents.
"Even on a hot day the temperature of the water is much colder than people expect so the body can go into shock very quickly and this can cause respiratory problems."
Brian Robson, West Yorkshire Fire and Rescue Service's senior community safety officer, said: "We would strongly advise people to stay away from the water, despite how tempting it may look in hot weather."