You are missing the point. Agreed, OWS is not the standard for testing GPS accuracy and it shouldn’t be used as “control” in any experimentation for all the reasons you identified.
HOWEVER, when the Fenix2 is off by MILES on four different occasions in an actually known OWS course distance and the 910XT is accurate to the hundredth of a mile on two different occasions, you have to say HMMMMM…perhaps the 910XT has better GPS accuracy than the Fenix2 when it comes to OWS. Same swimmer. Same stroke length. Same course. Same time of day. Same wrist. Just different watches.
When it comes to GPS, I assume some variation and inaccuracy. All you need to do to realize this inaccuracy is run/record a few laps around a track in a wide open area to see there are going to be some inconsistencies. Fine. In the end, I am only interested in knowing if I ran 10 miles, 15 miles, or 20 miles when I go out and run for time. I’m not really interested in whether it was 15.1 or 15.2. I know some are and some use it for pacing and racing.
When it comes to the swim, I think we can all agree that if I swam 2.5 miles and my Fenix2 told me I swam 4 miles, the Fenix2 is worthless in OWS. It’s not like it said I swam 2.7 or 2.8. That, to me, it acceptable error. Off by 60%? That makes the product garbage.
Testing GPS accuracy in an open water swim is about the worst possible test for accuracy you can do.
But it’s really good for testing the accuracy of GPS in an open water swim.
My point being that GPS accuracy can vary widely according to use-case. Just because a GPS is accurate in the open doesn’t mean it’s going to be accurate in an open water swim. And vice versa.
I’m 100% in agreement with rbuike on this one. Getting accurate OWS GPS with a watch on your wrist is a crapshoot, your arm is underwater most of the time, unable to get a GPS signal and dependent on the brief time your arm is above water to get a signal. Depending on your swim style, stroke rate, etc., accuracy is going to vary widely person to person. I’d guess it would even vary if you’re swimming on smooth vs. choppy water. If you want an accurate GPS track, put the watch in your swim cap. If you want your time and only a very rough idea of how far you went, put it on your wrist. I swam a measured course with a watch on my wrist and had nearly a 20% error rate. The concept of “testing” GPS accuracy with a watch on your wrist is just inherently flawed.