Open-water swim distance differs from the track recorded? (Todays swimrun 352m vs. approx 750m, unacceptable)

Same issue here and this is SO anoying. I've done 4 swims with the 955 and distance is always off by a lot, showing I'm swimming at 2:07 pace (I've never been this slow, even with bad open water conditions). Yesterday actually had my 955 on my left wrist and my old 920XT on my right wrist. I know the distance where I swim is always around 1600m. 955 showed 1511m at 1:54 pace and 920XT showed 1678m (close to reality as we had a high tide) at 1:43 pace which is what I usually swim at. 

Same issue here and this is SO anoying!!! I've done 4 swims with the 955 and distance is always off by a lot, showing I'm swimming at 2:07 pace (I've never been this slow, even with bad open water conditions). Yesterday actually had my 955 on my left wrist and my old 920XT on my right wrist. I know the distance where I swim is always around 1600m. 955 showed 1511m at 1:54 pace and 920XT showed 1678m (close to reality as we had a high tide) at 1:43 pace which is what I usually swim at. @garmin should do something about this!!!!

It's pretty evident from OP's activities that for some reason the 955 seems to fill in missing data or overwrite data with a pace of 5:15. It then mistakenly calculates distance with a 5:15 for that portion, or it doesn't calculate it at all when it sinks to infinite paces, instead of filling it in with the average pace between its gps datapoints.

This is clearly evident on parts of this activity such as the 2nd swim:

or the 3rd swim:

And this is from a friend's activity recorded today (1.2km total, recorded on Garmin Connect as 666m):

Garmin-Dantagging you so that this gets some traction, as recording only half of the distance for open water swims is a pretty big dealbreaker for someone who bought the 955 for open water swimming.

Same here: 1500m Competition distance and 856m measured! A qute expansive watch should be a little bit better then estimating!

So it seems that it does something stupid when it's not getting GPS compared just measuring the distance between the previous point and next point known? Of course those points could be wrong, but one can probably filter the points that are too far away from the last known position that one can swim and filter those out.

If they would just use the GPX for the distance it seems to be much better than this some algorithm they are using currently when GPS is lost.

I have the same problem with a Fenix 6x pro. Track seems accurate but distance is always way off and the pace.

what I see from the pace graph is the following: the pace during no GPS signal is taken by the watch with the value 1:54/100m that it could be more or less ok. The problem is the constant drop off on the pace during the process to acquiring and lose the GPS signal. This drop off brokes the final pace and the distance.

A example done this morning in a 1.500 m OW activity with the defected watch in one hand and another fenix 6x pro on the other hand:

and on the other fenix 6x pro in my other wrist:

If you see the pace in movement (ritmo medio en movimiento) both watches are more or less in the same values 2:08 vs 2:13. Instead on the average pace there is a big diference due the drop off for the first watch 2:11 vs 2:30.

Thus I think that the problem came from that data. The drops off of the second watch, are right, and there are due a change direction during my swimming.

I ordered to Garmin a replace for the first watch because I couldnt fix it removing and restore factory data as well. I was lucky because still have the legal warranty but I think a major issue I dont know if it is with software or hardware.

Losing the signal most likely is unavoidable so it's more just how to handle it in software. So would not think as HW problem but as a SW problem. Also based on the experience with 945. When 945 came it was also very crappy in OW but then updates made it work again. Even more bad than 955 has been, as it lost totally the swims and distances were nonexistent. 

Ref. https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-945/164018/945-appears-to-have-lost-gps-32-metres-into-open-water-swim 

If it is a SW problem I wonder why it isn´t solved by the factory reset

You can't fix bugs by doing factory reset. The algorithm/handling is flawed and continues to be so.