I had the same problem yesterday for a 500yd lake swim. I would get GPS out of the water but never did get signal when I was swimming. However, for an outdoor pool swim I got GPS signal fine two weeks ago and for an open water lake swim a week ago I got GPS signal fine. So I don't think people are correct when they assert that water blocks the GPS signal. I believe it to be the clarity of the water. Yesterday during my swim there was zero visibility. Just my opinion though.
The timing just turned out to be lucky for you in your latest outdoor swim because Garmin specifically states that GPS (on wearable devices) does NOT work underwater.
I have the Descent Mk1 and it saves my dive locations (enter/exit points) but only if my arm is above the water when the GPS is running. It has nothing to do with U/W visibility.
Fenix 5 open water does track any more. 2 week end ago, it tracked 40% of my swim on a 1 mile swim and then 50% on the next one mile. See https://connect.garmin.com/modern/activity/3912158375, no gps but strokes
Today it tracked 38 feet for at least a 1000 yards swim. It captured the strokes though see connect.garmin.com/.../3965596112
I then switched to walking an the GPS works. It is not as great as when the open water swim option worked, but it is far better than the previous one from today, See connect.garmin.com/.../3965596425
There are bugs in the latest software update. I have two race coming, please fix ASAP. Thanks. Jerome