I found this in the London Evening Standard for 22 January 2015, by Charles Saatchi.
Two thirds of victims of death by drowning are decent swimmers. And the majority of fatalities occur less than five metres from safety.
Apparently, panic is the primary reason, as even proficient swimmers experience "instinctive drowning response"; this follows an anxious moment, with the classic surface struggle leaving them floundering, thrashing in an upright position, with an involuntary loss of control.
Swimmers in trouble have little chance to shout for help as they gasp for breath, though they are often near others who have no idea that they are watching someone drown. Often, they will appear to be simply treading water, presenting just a blank stare. There is usually little to be seen of fevered splashing, waving or yelling after the first moments, just as the involuntary loss of control leaves you unable to keep your head above water.
It takes less than a minute for someone drowning to disappear below the surface, and then usually another minute before they are forced to inhale water, blocking the lungs and creating spasms in the larynx before they swiftly die. Strong, fit swimmers often underestimate the difference between swimming in a pool and in the sea or a river or lake... A lifeguard explains: "When people hit cold water, their muscles react like stepping into an icy shower. And having a buddy gives a false sense of security because a rescue in open water is problematic, and your companion may have his hands full keeping himself afloat."
Charles Saatchi, the infamous wife-strangler, knows a lot about people gasping for breath, but does not give a source for this information about accidental death; however it sounds true to me. Just to recapitulate the theories about what happened at Sandford Pool: Michael and Rupert drowned because (1): they planned to commit suicide together, (2): they decided to die together on the spur of the moment, (3) Michael dragged Rupert down, (4): Rupert dragged Michael down. We can now consider number five: that both boys panicked and effectively dragged each other down - which I find compelling. Still, in the wise words of the Eton Chronicle "We will never know what happened that day."