15.19: open water fixed or not?

One can compare the open water swim profile with other profiles e.g. run or walk. Just take a route around the park and record it once with the walk profile and once with the OW swim profile. It is clear that the OW profile applies some heavy smoothing to the GPS track. Corners that you didn't cut will be cut quite heavily. I think that contributes to the OW distance being inaccurate, but it might not be the only reason.

If that were the issue, the distance on the GPX track would be short. But it's not - it's only the distance in the watch statistics.

Sorry for confusion. I think that the smoothing could be applied in Garmin Connect and potentially on the watch itself. I was referring to GPS track as seen in Garmin Connect. The underlying FIT file with GPX track might not be affected. I'd need to do more testing to confirm.

I've never heard about any smoothing that can make a curve longer? 19% longer in fact

For me distance seem to be ok. But pace is absolutely inaccurate. It changes from about 1:30 to 1:55 min/100m every few seconds. The graph in connect is horrible.

for me too the rhythm is imprecise. i swam the same way over and over keeping a steady pace and got a horrible graph 

My OW gps track is horrific. It’s in full multi band, and when compared to swim buddies fenix 7, might as well be in a different place. Completely dreadful. 

my GPS track is perfect. it's the rhythm that's completely wrong.