I mostly do OW swimming, but up and then it happens I have no better choice than swimming in a pool. The pool length detection is not perfect, but it depends on your technique, and especially on the technique of your turns. Up and then I have to hit the wall with a hand to get the length counted.

As for the hardware - there is no difference between Swim 2 and Instinct 2 - they both have only the accelerometer, and that's a pretty basic, and a reliable component, which cannot be blamed for any inaccuracies. That's rather a problem of the software that desires some tuning. There is perhaps a slightly better chance to get better results from Instinct 2X, since unlike Swim and Instinct 2, it has also a gyro sensor, which could theoretically help with the turns detection, and with swim-stroke detection.

The question is, though, whether the software gets any advantage of the gyro at all - I do not know that, and cannot compare both devices simultaneously, which would be the only way to tell. Perhaps another user who upgraded to 2X, and still kept the smaller model, could do a test, wearing both watches during a longer pool swimming session.

I’ve swam, and still do occasionally, with the I2 and I2X without any issues counting lengths in the pool. However, I only swim freestyle and usually in an uncontested pool where I can swim freely. My fumble turns aren’t the best and I’ll more often than not do a sort of half fumble turn or simply turn with my head out of the water. If you’ve had problems with other watches when pool swimming there’s no guarantees that you won’t have problems with other watches. 

Thanks for so quick answer!

I was using Garmin Swim (no number, just the 1st generation) designed for pool swimming only, and it was great. It was counting lengths correctly, and one CR2032 battery lasted for ONE year (simple LCD screen, not any unnecessary features like GPS, smartphone cooperation). Made for swimming and is perfect for swimming.
After a few years, it naturally has gone (type of corrosion) and I invested in Garmin Swim 2. It was only a few days on a battery... hard to accept after that one, but they advertised it as a specialized model for swimming - I took the risk.
My swimming style was not changed but the measurement of it by Garmin - yes. I have never made a flip at the end of the pool, just stay for a while, slightly hit the wall by hand and push my feet off the wall. It was enough for Garmin Swm (1). With Garmin 2 (I use to swim alone, no other people in the same lane) only about 70-80% are 100% correct. It drops length, it adds lengths and hard to find a rule, find a way it works. It's strange, especially when I use the "auto rest" function that recognizes when I stop - indeed it works correctly in this area. I mean, I notice that the watch face changes to rest mode and shows the countdown for rest time. But suddenly a length disappears.
Garmin Swim 2 is from 2019 so asking for a newer Instinct I was asking also about (the new one?) algorithm which is used to correctly count lengths. Maybe they were working on it...
So, still looking for the answer if newer models (Instinct 2, FR 255, FR265) are worth buying :-)

As I wrote I am not a frequent pool swimmer, but had a similar experience with the accuracy at Instinct and at Instinct 2 - it kind of works, sometimes I need to tap the wall stronger, but the accuracy is not rock-solid. Good enough for recreational swimming, but I am not sure I could rely on it for any serious training, when I'd have little time to survey the watch and eventually try to fix any problems with it.

On my mind all recent Garmin watches use the same algorithm, but that's just my guess based on the feedback at different models here on the forum (though I only follow the models in the Outdoor section of the forum), and based on the modular way I see Garmin uses at the development of firmware of individual models. As written, I also do not know whether the presence of the gyro sensor at some models helps or not. Theoretically it could.

Using the HRM-Pro chest bell could theoretically help too, but that's just another speculation - I do not use it often at OW swimming, and never tried it in a pool yet.

You could try asking on the Forerunner 945 forum, perhaps it behaves more reliably in pool. I saw it ranked high in several comparative reviews for swimming (although I did not find any really trustworthy comparison going sufficiently deep into such details as the turn detection)

You probably are right in the opinion that Garin uses a modular way of making their watches because I also asked on FR255 and FR265 forums and people who own that models say that counting laps on the pool is not perfect...

Proceeding of swimming activity in the pools did seem to me not too complicated. And it was not complicated when I was using Garmin Swim (1) for a few years - maybe one-two incorrect count a month. Last day my Garmin Swim 2 displayed a "new record" after swimming, making my speed on the Michael Phelps level :-) Of course by adding me laps (and the time was as it was). So, not only the swimming technique could be a problem, but having all my history the watch should calculate, that swimming suddenly so fast is just impossible (2x faster with 2x fewer strokes...)

It stands me on the ground to look for some other.. maybe Suunto? But this is for a completely another forum :-)