I don't mean to sound dismissive of this - it is a horrible tragedy. The sad thing is that here in Phoenix not a week goes by that at least one, usually two or three kids drown in pools.
Over the years they have enacted laws that say all pools must have their own locked fences even within backyards, etc. But it still happens.
Kids can drown in no time and they do not make any sounds at all. Like that video that was recently circulating showed, a drowning person does not have the strength to yell or wave. Their head usually is bobbing at the surface until they lose whatever air they had in their lungs and then they sink.
It is odd but out bodies are generally composed so that when we have air in our lungs we can float but when we lose it we sink. You have to use some energy not to drown (swimming) so people should be taught how to swim at a young age.
Most kids who drown are younger, though. It is hard to explain why an older kid would drown but I have seen it happen. Probably the #1 killer of kids here in Phoenix.