Strange swan behaviour

The other day, I saw a juvenile swan behaving in an odd way. It was lying flat out in the water, with its neck outstretched and wings outstretched. I was alarmed, especially as this was happening right in front of a group of men fishing. I stopped and asked the men what was wrong and they shrugged. One said they had poked it with a fishing rod to check if it was alive. The swans family was nearby, siblings and parents, and they didn't seem distressed at all. I started to call the rspca because I was worried but after a couple of minutes, the swan suddenly sat up, shook itself off and swam over to its family, apparently fine!

I wondered if this was a sign of over heating or something? It was a hot day and the family were swimming about in the sun but the juvenile was flat out in the shaded bit.

Would love to know what happened here. I am a boater and live on the Canal, so see a lot of bird behaviour but have never seen this before.