I use the in-built apps on my Garmin and that automatically uploads the data to the Garmin Connect web site.
The web site and the iphone/ipad apps give me average speeds and general stats. The Windsurf Garmin app gives 5 and 10 second averages. You can zoom in on sections of the speed graph and see what is happening at a small scale. If your speed has doubled over one or two seconds then you should be suspicious that the peak is an error.
I don't trust max speed from any watch based app. The technology simply isn't accurate enough to give a realistic number. You need much better devices and use the various error correction infrastructure to remove the errors.
What I do is export the GPS data from the web site in GPX format, then use an app like GPX Viewer to see the data in a table format. From there it is easy to copy/paste the data into Excel and slice and dice the data any way you want.
I usually sort the speed data into speed order and plot it onto a graph. You end up with an S-shaped curve. It's easy to eyeball it and exclude the high and low peaks leaving you with a realistic speed range in the flattish part of the graph.
These graphs are from a wingding session. No speed runs but heaps of extended glides on chop, and a few falls.
Raw speeds from the GPS:
Sorted. The left side is hanging in the water. The bump on the left would be gybes. The right side is blips up to 20 knots which may or may not be plausible.