Dumb Question

Wow, big questions!
Mainly it's not so much speed over a certain distance, but average speed over time.
The 2 second average is the fastest result so seems what most people concentrate on. But there's a feeling that such a short time can be a fluke, so the average of the 5 highest 10 second averages is considered more an indication of consistent high speed sailing.
There are a few other divisions we also compete in

The site I use is GPS Team Challenge it's an Aussie site that's now gone International.

www.gpsteamchallenge.com.au/index.php?country=0

As the name says it's a state based team competition, but also keeps track of individual scores, so there's also and individual comp.
The comp is in 6 divisions, 2sec 5X10sec, Nautical mile (here is where average speed over distance does come in),
Average speed over an hour, distance sailed, and alpha which is a "U" shape where the total distance is not greater than 500m and the separation of the start and finish points is not greater than 50m

It was started as a State based team competition, but also keeps track of individual PBs.

This is the Dutch site.
www.gps-speedsurfing.com/default.aspx
That is for Individual scores, and keeps track of the above divisions as well as speed over 500m

And this is another Aussie site that analyses your data and gives results in the above categories
ka72.com and can upload to either of the above sites.

As far as I know non of these results can be approved by the official water speed body.

Same for wind speed to knots, we'd all like that, but nobody has come up with a practical way of doing it.

The GPS Teams Challenge is primarily about building up links and a community between windsurfers in what can be a solidary recreation where at best you may only know the first name and the car they drive of people enjoying the same pastime as yourself. The Challenge has been a great success at this. If you want to get to know other windsurfers around and you are into flat water windsurfing you should join a team in the Challenge.