The garmin watches come showing your mile pace, but you can easily change that to kph or mph. Showing av over the last 500 m seemed to work well as it even's out the errors in instantaneous GPS speed. Never had a problem with the zig zagging. But... for sailing I don't think the £200 watch adds anything (in fact, i think for running they don't really add much past a regular phone). 

As for a site which logs the data, I just used Garmin Connect, but I see some people using Strava (much better social community and one I use for sailing). 

However, I think there is a massive hole (well, sailing isn't that massive, but I'm sure there would be interest). for  sailing specific site. One of the best things for Strava is for races there is a 'flyby' feature. Its takes everyone who did the same course as you that day, and 'plays back' all their activities. For cycling it's quite interesting seeing where breaks form, where the peloton split, where the lead groups made their gains etc..

...but for sailing it would be even more interesting. You could see exactly where they sailed, aver speeds for legs, VMG or replayed sped up or in slow motion. You can just use smart phones, so most people already have the equipment required.

As an example here is Steve Cockerill and George Yoeman at the Chichester Snowflake, shown in 'flyby' on Strava. 

http://labs.strava.com/flyby/viewer/#880380081?c=gcp3hbuy&z=F&t=1OigPE&a=sYh5NCLAfDQ" rel="nofollow - http://labs.strava.com/flyby/viewer/#880380081?c=gcp3hbuy&z=F&t=1OigPE&a=sYh5NCLAfDQ

Unfortunately they were in different fleets, but you could filter based on the time when people pass through the start line. You could then have it replaying in the bar after sailing. With a few tweeks to show more sailing specific data (VMG / speed in knots) it would be awesome. 

I've used raceQs for that. You have an app on your smartphone that records your position, and then you upload the track to the site and you can see an action replay on a Google Earth map. Quite interesting, but as I was the only one doing it, it didn't turn out to have too much value besides getting used to sailing in straighter lines. 

If more people are doing it, you can see them all side-by-side on the course.

I think something like the strava flyby, but in a more sailor friendly metrics would be more interesting. Stuff like showing VMG for each leg, knots as the speed, overlain admiralty charts rather than google maps. Overlay meteorology data. 

Basically, the technology is all there to produce the sort of tracking you see for international regattas and volvo ocean race etc, without the need to buy sailing specific tracking equipment.

I think it would be a excellent learning tool for coaching, but also a bit of fun in the bar afterwards: who was fastest of the reach? Who points the highest up wind. Who snuck around the windward each lap without ever getting that last knock?  

It's added a lot of value in sports like running and cycling and I think it could be even better in sailing where it's often quite hard to visualize where and when the race was lost and won, 

I'm surprised none of the sailing companies has made a go at it? Maybe for the small market it just doesn't make sense to invest in a low sale value product (an app) as the numbers who would buy it just aren't there.