open water swim accuracy on the instinct 2S ?

Does not happen to me, but I have Instinct 2, not the S version. Check out the document Tips to Improve Accuracy of Open Water Swim Activities, perhaps it helps. Especially make sure you synced with Garmin Express not too long before the swim. That assures you have the latest CPE (ephemeris) data file, assuring the quick GPS lock.

I have recorded several open water swims with my Instinct 2 without any problems. My wife has swum once or twice with her Instinct2S and they’ve recorded ok. First question is what stroke are you swimming? Best results are from freestyle. Breastroke doesn’t work as the watch is under the water and unable to get gps signals. 

thanks for that , yes doing purely free style and on this particular swim I kept my arm over the water as much as possible , which actually improved the accuracy a bit , it seems that the watch is not pinging the Satellite every second as it should , I will tie the watch next swim to the float and see what happens 

thanks for that , had been looking for that but couldn't find it until you said CPE File , yes mine is current, will do more testing 

Brittal12 said:

yes mine is current

Unfortunately, it often tells "current", because the date is all right, but the file is broken or incomplete, because the download was interrupted. Updating it over USB from Garmin Express is the only safe way. Relying on the automated update from the phone over Garmin Connect fails far too often.

Change the reporting mode in device system setting from the default Smart Mode to Every Second Mode. Then go to the setting of Open Water Swim activity settings, and set the option GPS + Galileo.

the issue mostly resolved over time , for a while mine was the most accurate swim watch in my swimming group ( my mates swim with Fenix 6 ) I did a couple of swims with the float and later on again on the wrist and either firmware or luck it resolved , but yeah give it a go with a float that way you can be sure that the watch is above the water