Clearly no one here has ever been near the Bournemouth or Boscombe piers when its hot. Locals tend not to go there as it is ridiculously rammed with Grocks, this is the same place that girl was raped in the sea.
I doubt Police know exactly what happened, they were called by the ambulance so they wont be saying anything until sure otherwise the usual will start on them if they say something slightly wrong.
My opinion, for what it is worth, is that with kids jumping off the pier (a very dangerous thing to do) they have come into contact with something like a boat/jetski or windsurfer. Sadly these people often come in too close and in this case may not have been expecting swimmers so far out. I may well be wrong of course.