It says serious medical incident, which doesn’t sound like the little boy drowned.
Even if he did drown, it’s a tragic accident not anyone’s fault. It could happen to anyone, accidents happen all the time - they could go under in the 30 seconds you look away, they could slip and hit their head, they could step in the road and be hit by a car.
Anyone who blames a person for the death of their child is a disgusting person and should have a good look at themselves. If your child has ever injured themselves at home, you are not perfect and the injury you ‘allowed’ to happen could have been a death were you unlucky enough. Accidents can’t always be avoided and compassion and sympathy is all that’s needed in this scenario.