You can get heart rate while swimming with the optical sensor, if you select activity other than swimming on the watch, because otherwise the optical sensor is disabled. Chest straps of any kind won't work with the 645 for swimming. Best solution would be to get a watch that has swimming mode and works with the HRM-Swim and/or HRM-Tri chest straps.
Yes, unfortunately you need a triathlon/multisport watch for swimming with HR (from chest strap). Current tri watches would be 935 and Fenix 5/5+. There are also the older 920xt, 735xt and Fenix 3.
Optical HR is disabled during swimming because Garmin doesn't think it works very well underwater.
The Polar H10 won't work for swimming with Garmin in any case because: - It uses an old analogue frequency for live transmission which only works with Polar watches - It does support "caching" of HR data to be sent to the watch after swimming, but only with Polar watches. (Apparently there's no standard for this, so you can't mix and match manufacturers)
You would definitely need an HRM-Swim or HRM-Tri chest strap to go with that tri watch. I think the HRM-Swim is recommended for pool swimming (non-slip design) while the HRM-TRI is good for triathlons. https://www.dcrainmaker.com/2015/11/...ri-review.html
