bad accuracy for indoor swimming activity.

Well, if we have same watch with same hardware and only your can't count correctly it is clearly a problem in your style

Miky, are you an Olympic swimmer? There is clearly a problem with this watch since I never had a single issue in 10y with any Garmin watch. I was a swimmer as a kid, and after that I was a water player for almost 15y and I always was the one who swims for the ball at the beginning of the game. I swam 55min for 3800m in Ironman race, so belive me I know how to swim. Day before yesterday I had one-hour race pace time trial. I was counting laps since that session is super boring. I did 3850 in 59:40 and garmin showed 4200!!! So that's obviously really bad. And no, that has nothing with technique because the effort isn't so hard so you can really do it the proper way. Your watch is OK, but mine isn't.

for what it's worth... i found my good-old garmin 935 ,, charged it,,, swam 3k with 'my so called lousy technique' .. flawless... dead accurate...  I'm not an olympic swimmer, and yes, sometimes i turn my wrist when pushing off the wall to peek at the watch... i'ts what - i guess- most people do that buy these expensive watches. They are not only made for perfect swimmers (i hope)

I'm not perfect too, rarely I stop for 2s and turn my wrist to read the watch, just today my watch counted 1 more pool over 100 for maybe second time in about 50 trainings.

Maybe your watches are faulty

It worked for years without a problem (935, 945, 945lte, 955 until recent software upgrade). I think the sensitivity is just too much. Funny, but there was an ex-olympic swimmer in my lane today.. I will ask him to wear my 955 if he wants to do that next time :-) .. And at the same time 'look at the watch like a middle aged, middle-of-pack AG triatlete  :-) )  .  I'm going to file a support case indeed. Maybe the'll replace it and i'm lucky.

I stopped doing this (unfortunately), from the moment that I found out that this movement resulted in adding a full pool length. As you mentioned, this is a simple usual movement we all are used to do to check a metric. 

I also have this issue. Today I have swum 48 lanes, however, according to my FR955 I've swum 72 lanes.

I checked in Garmin Connect if it is detected the correct technique, and it does, except for 1 lane.

The time per lane should be around 50 seconds (I know, not very fast...), but my FR955 recorded it often as 15-20 seconds per lane.

Allthough the pace is low, the technique is good.

I'm on 15.11 and unable to update to the newer beta. It says downloaded, but that's it. If I try it again no updates were founds

Is it btw possible to show the number of lanes on the watch and app, instead of the distance?

Couldn't find any option related to that.

2 months ago I bought 965. Not a single issue regarding this problem. So it’s absolutely not my swim style, it’s about the watch (sw in it)

haha,, my experience exactly :-) Just threw down 650 euro for the 965 and it works perfectly (just as my old 935).