Lets think about device that generate sound at the pitch proportional to the difference between wind speed measured at the top of the mast and real GPS speed, or relative to the water speed.
Electronic or mechanical. quite easy to implement.
In practice:
Now you have two windsurfers (or sailors in general) running in parallel at the distance. One is running much faster then another. But his sound pitch is higher. Which means that is not especially more effective in catching the wind , but just cough the gust.
Drawback:
Each windsurfer create additional artificial noise that for recreational sailing could be annoying to one.
For recreational moto bike rider or jetski - that is not a drawback at all - the noisier the exhaust pipe - the better[}:)]
Pro:
Such tool could be helpfull for racers to tune up their gear and position in fly..
Design:
The most rudimentary could be a vertical axis turbine + small ac generator -that spin at speed proportional to the relative wind speed - then generate sound using buzzer ( small speaker) or mechanical simple clatter
Or there is already such thing used by all yacht racing crews 
Implementation
tiny anemometer installed on the top of the mast ( like that but with some mesh protector around ) connected directly to piezzo buzzer or speaker will do the job.
Drawing 
Other>
*For 3rd observer (coach) take also Doppler effect into account when comparing this two competitors.
**You know guys what? By publishing such idea I may void any further claim anybody else may have.
Which translate to : quicker to the market and cheaper for all of us .
Free to all to use 
like Android
***Electronic more advanced version could compare data from other sensor: GPS, vacuum sensor , etc to compute more accurate results and sound/display/transmit/record output
**** It potentially increase safety on the water, by making other aware of approaching sailor
* As for kiters, similar device could be easily mounted either on the helmet (more accurate) or on kite itself -more fun when turning.
* Now visual + audio effects could rise attractiveness of the sailing sports ie Olympics, racing for spectators
*for simplicity we could call device SCREAMER -Sound Creating Reactive Emitter Analyser Mobile and Enviroment Resitent - other community proposals welcome 
)
If I was Apple (R) I would quickly patent the way the speaker is mounted to the mast or the colour of the winglets to have all royalites then
I like the sound of the wind running through my hair (on my ears)...not a whistle or buzzer - no-extra noise for me thanks.
What about a low tech idea
a flourescent rubber d!ck on a helmut with a whistle in the nob - the faster and more efficient you sail the more erect it becomes as it catches the wind and the faster you go the louder and higher the whistle gets
then the coach can see it from the beach and make comments like -the noisy d!ckheads really fast and efficient today
reminds me of jet skiers at my local beach
Run a magnetic pulse paddle sensor behind the board
www.georgfischer.com/en/service/errors/404.html
The faster you go the higher the frequency of the output signal.
Wire it though some sound gigery pokery like a guitar distortion peddle then hooked up to a speaker.
Get it going a warp speed and annoy all the nearby dogs
I just listen to the crowd on the beach screaming.
Once it is a crescendo, I know I'm doing good.
The worst thing in life is to disappoint one's fans.
Might move over to kiting soon.
You'd need a combination of an accurate GPS unit, a reading of the apparent wind speed and direction at the top of the mast, and something to do the on-the-fly claculations and make a noise. Perhaps an iPod.
A few vectors later you'll have your answer!
Note with the below example you'll actually have apparent wind and boat speed, so you'll want to first calculate true wind speed and direction before you apply it against your boat speed and direction, to give you your "efficiency".
An iPod could talk to your GPS (probably not) so get cracking on iOS development.
As you said, "quite easy to implement", so I'll leave the rest to you. We'll be expecting a prototype any day!
like Android
^ What about a Heads Up Display type configuration in sunglasses then? Quite easy to implement.
I accept all critics by merrit , but please do not attack me now for spelling etc as I am very busy now with my gardening ( there is no wind second week )
As Evipanda says you definitely need direction as well as speed. This won't be easy as the mast tip is not at fixed orientation. The mast can tilt and also rotate about its long axis. There's 2 more transducers that are needed. Or your could gyroscopically reference the anemometer?
Steady winds are quite turbulent. The output will be a warble, the speed of the warble will depend on the mechanical inertia of the anemometer, or the electronic damping applied. The brain is pretty good at integrating a warble and picking the mean frequency I suppose. But a bit of work will be needed to best tune the response. Too little damping and the warble will be indecipherable, too much and the board will have accelerated on gusts that have been filtered out.
Good idea in theory but i think it would be a considerable engineering feat to get right.
We'll be expecting a prototype any day! if anybody else will not do this earlier. I strongly advice , anybody can go to garage and do prototype in half and hour if have got some parts:
agree) i just type without watching keyboard or screen, just thinking... )
Hello guys , I don't need anything for that. You could do quick patent in SB name and share or spent profit to publico bono - as you wish.
The patent could be obtain for device itself or even same idea of presenting specific measured component in audible form, more to imagine here that in device itself
Or do yourself and sell without waiting for Chinese competition.
wait ,
no,
you may give me a discount on new slalom board when come to your shop
Your GT-31 already beeps in (near) realtime, to your actual speed. Maybe you could just listen to it.
