Hi Kazza,
I assume you did not see the previous many threads about the Garmin and Suunto GPS sports watches on Seabreeze.

The subject has been explained at length by GPSTC and argued over by the same small group of individuals who continue to choose not get it I'm afraid.
In a nutshell, the issue with those devices about consistency and verification.
The decision was taken long ago to only allow posting from GPS deviced that record the Doppler speed error data so as to make it possible to very effectively verify the results as being real. We have seen some exceptionally false results from many devices in testing, even some of the approved ones, but at least with the approved ones we can verify that they are indeed false. No way to do that with any confidence in those other devices unfortunately.
But there is a factor of accuracy here as well. Virtually all the 'watch' type devices (except the GW-60) do not have room for an decent antenna and this does affect accuracy, consistency and error significantly. Even the smaller patch antenna in the GW-60 is marginal in it's reception and there are instances where in low satellite number availability, and with the less than ideal, compromised position on the wrist, it has ceased to get data during sessions. This did not happen with the GW-52 and GT-31's used at the same time, and certainly not the Motion which used up to 3 satellite systems concurrently (up to 27 satellites at any one time).
Many people do like the user convenience of the GW-60 in spite of it's shortcomings, and accept that it may have a limited lifespan (not unlike sails, masts , wetsuits and boards etc??) Others have found that theirs have lasted for hundreds of sessions and are still going well. I must say I have been very disappointed in the warranty backup and service attitude of Locosys re the GW-60.
The new Motion GPS is by far the best device for windsurfing we have seen, tested and used so far. It is a one man initiative by a very clever enthusiast in Belgium and he is very keen to solve any issues and is great on warranty and backup service. Because it is a one man operation he has been somewhat overwhelmed by the volume of orders and is taking steps to streamline and speed up his production. There have been almost no issues reported on the reliability or functioning of this GPS, but a few other issues that have led to improved wearing security, hence the redesign of the arm strap holder. There is a comprehensive report on it somewhere on Seabreeze from about a year ago when we went through the approval process.
In theory, it 'may' be possible to test enough (at least half a dozen) Garmin watches side by side, for hundereds of hours and 1000's of kilometers, to write firmware for them which would give us the vital information we would need for approval, but the manufacturer has shown zero interest in this and nobody else has the resources or access to the firmware to do it. It is extremly unlikely to ever happen, and is certainly beyond the ability and resources of anyone associated with the GPSTC.
I hope this helps.