Scary...but that reminds me needs for
100% obvious emergency SOS signal (distress ) on the water.
We in our windsufring family did such aragements regarding special signs but could be
nice to have one universal recognizable from kiloometers away that person needs help now
Unless I am missing something and there is already such hand singnal 
For as long as I can remember the in water distress signal promoted by surf life saving has been to hold up one arm above your head and wave.
In that case even a distress flare wouldn't be much use - you need two hands to operate them.
Fortunately if you're in the water for long someone usually comes by to see if you need help.
I have been thinking about emergency signal for a little bit. The best option I could think of was an emergency whistle. You would think there would be a smoke flare that you could use with one hand but I havent been able to find one.
I think your also supposed to have a closed fist when waving so as not to be confused with a friendly hello.
The internationally recognised distress signal is waving both arms so as to cross over hands at face height. Hmmm - imagine doing actions for YMCA (yes a bit ghey, but anyway...) Make the "Y" . Now make the "M" without dropping your elbows. Repeat.
That has been the international distress signal for god-knows how long as waving one arm can be hello. It also is about the most movement you can make so is easily recognised at a distance
How can they not teach that in WS and kite school..........
guess that was his lucky break to get onto you tube..
handy video...
Rather not see any nasty downsides to going fast, or hard....that was ouch big time
Over here in wa every were ive sailed we all keep a look out 4 each other allways because you never know when it will be you next its a unwritten law of the water weather your surfing windsurfing or tea baging thats what makes you feel safe rescueing someone feels better than a good sesh on the water i think thats one of the ingredents that makes us ozzies unless you have got your head up your ar#e then karma will take care of you !!!!
There is only one signal to remember... the "ok" signal - fingertips touching your head so that your elbow/arm makes an O shape.
All other signals are "get the F*** over here".
EDIT: the ok signal can be used as a question and as a statement, and you can see at least about 100m away.
Floating face down in the water is universally recognisable as a distress signal.
Use whatever you can, if you can sit on the board wave both hands if practical. When I had my little mishap last year was either holding on or swimming so two hands would have been out, hell, wave your helmet, rashy or harness if you can, I'm sure that wouldn't be confused with hello.
Kiters have this edge over us for sure 
They could always drawn their yellow red dot polka boardies over their head and that is umistakleable sign that kiter is in distress 
gets attention too
You wouldn't think water was that hard, but then 10 metre tower divers who hit the water at sqrt(2 X 9.8 X 10 ) m/sec = 14m/sec = 50.4 kph often use wrist braces just in case they don't break the water exactly as planned. ma was doing 75kph!
Melissa Wu and Alexandra Croak doing the 10m synchonised dive.
so maybe we should punch the water with close fist when landing ? Is it safer ?
the speed of a punch thrown by an average person :
40-50 mph, hits with 200-300 pounds of force,
and has a reaction speed of .2 to .3.
Bruce Lee was supposedly clocked at around 120 with a backfist strike,
I only guess nobody will hurt himself if puch the water the strongest he could
calculactions are correct: 14 m/s (jump from 10 meters )
U^2 t V ^2=2as
u - initial velocity
v - final velocity
a- acceleration
s- displacement ( our 10 meters )
