I've got two HRM-swim monitors brand new in the package and won't use them because the heart rate from the 945 seems believable.

I can't think of a way to compare them back to back.

Ditto above. I do not have an HRM Swim, but the 945 seems totally inline with RPE and very old Scosche Rhythm24 data. And, the new 745 does HR swim too.

Great question. I have been using the Garmin swim HRM for a few years and have gone through 2 bands. I got the HRM Swim when I bought the 920xt. I bought the 945 a year ago and have still been using the band during swims. Garmin put an update Last December into the software that was supposed to make it more accurate. DCRainmaker did one of his awesome updates about it here. https://www.dcrainmaker.com/...e-gets-profiles.html. If someone knows how to keep both readings, there is a way to compare them through Kubios (https://www.kubios.com/hrv-standard/) . I got this information from Alan Couzens on a different thread.
Makes me wonder why I am still using the PITA band!

DC Rainmaker's comment on the new 745:

Finally, a word about swims. The FR745 will utilize the optical sensor during swims, both pool and openwater. In my case I’ve done a number of openwater swims with it – more on the accuracy later. But in short, for ALL watches on the market today, measuring your heart rate optically with water between the sensor and your wrist is incredibly difficult. All companies basically say a variant of ‘Good luck, it might work’.

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"...the street finds its own uses for things"

I've been using a Polar with optical HR for about 18 months.
So not directly comparable to a 945, but some similar principles with problems with optical.

When swimming, much of the readings seem plausible (with the usual caveats about optical having delays etc).
BUT I've found sometimes I get V high readings. Like near flat out 160+ or even 170+ when my HR is defo NOT at that (when on a steady Eddie pace that I'd expect a HR of more like 125 or 130).

I was looking at this just a couple of days ago, and have a pet theory (but not done enough analysis to prove it, and it could just be coincidence, or BS)... I think the watch is detecting and adding up 2 sets of info at times.... (a) the real HR, and (b) my stroke rate. I'm thinking the hand/ wrist is getting the rush of blood each arm stroke and the watch OHR sees it superimposed on the real HR and thinks 'ahhh that's 178' when really is 142 hr + 36 strokes).

There's no way I'm at 170+ for the length of time its suggesting - my absolute max HR on an FTP test and ready to puke is about 178-180.

I've had good luck with the wrist optical while swimming. It seems to vary from individual to individual so YMMV.

The wrist-based HRM generally works well for me (i.e. matches a chest strap). The exception is when I'm biking out of the saddle. For some reason that wrist motion causes it to read low, but my swim stroke doesn't impact it.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Was way off for me, but it is also when running.

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Let food be thy medicine...

If you have both you can use this Connect IQ app to record both and compare

https://apps.garmin.com/en-US/apps/a461bc0d-828d-47c8-a4a5-8d014b5fedc9

Quote:

Note that you should not have the external ANT+ heart rate monitor paired to the watch in the usual fashion, the data field will make its own connection.

Glad you asked this question: I've been thinking about upgrading to a 945 from my 935, and would love to ditch the swim strap (it just looks awkward, and I'm not a fan of how tight it has to be). I wear an HRM-Tri for running, cycling and racing, and will be upgrading to the HRM-Pro soon, but would love to get rid of the HRM-Swim. My wife's Apple Watch Series 5 seems to do pretty good optical HR in the pool, so the 945 should be on-par (I would hope).

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- John
"Have courage, and be kind."

Yes, as far as I can tell, the 945 HRM is the same as the AWS5 in swims. The standard YMMV caveat applies: that optical wrist HR is user dependent. It works great for me.