Hi,
I'm the developer of Open Water Swim, and I have had multiple reports and have also confirmed with my own testing that the WatchOS 10 update has seriously degraded GPS data during open water swimming, both with the native Workout app and with Open Water Swim. The data provided by CoreLocation is no longer accurate or trustworthy. I will post several pictures showing before and after update swims:
Here's the raw data (mapped) of a swim done with watchOS 9 on the Ultra.
Same swim today:
From another pair of users: These two went TOGETHER, one with watchOS 9 and one with watchOS 10. You can probably guess which is which.
Watch OS 9 swim:
SAME SWIM WITH A PARTNER ON WATCHOS 10 (beta here)
And another, again, two people swimming together, one watchOS 9 and one watchOS 10.
watchOS 9
Same swim, same time, watchOS 10
I have filed two feedback assistant reports with APPLE. One when it was still a beta issue: FB12587468. This one has zero response from apple.
And now a second one filed with watchOS 10 release: FB13192409
Apple devs, I know you're here and listening, so please let's get this fixed ASAP. Thanks