From a jetty into (sheltered) sea, so about a metre or so. Adolescent boy. Can't swim. Fully clothed.
I thought this would be easy to find, but there's too much noise (falling into the water is one of the most common causes of drowning... also dream interpretations, gaming pages). I've lots of useful stuff on actual drowning but not the prelude.
ETA: I think I replied to everyone. If I missed someone, thank you! Now I have to go and rethink what I've written.