It nice to see Garmin activating optical heart rate for at least their newer devices. It won’t be perfect, but it’ll add at least a little bit of insight into your pool session’s intensity.
I’ve haven’t tried it on my Forerunner 245 Music yet but I’ve also got a Polar Vantage V that does wrist-based oHR while swimming and I found it “reasonably” accurate. I’m a rather mediocre swimmer and the recorded heart rate seemed to agree with my RPE for the session. No really odd spikes or dips. On the other hand (no pun intended), the Vantage V’s swim mode is horrible compared to Garmin.I suspect that, just like cycling or running, if you’re really training by heart rate (which is not typical for swimming), you’d use something like the HRM-Swim. If, however, you just want a little extra insight into the session’s intensity, the oHR may be quite sufficient.
At this point, it looks like it’s limited to beta versions of the firmware for the following devices: