The HRM-Swim will work for any type of activity, however due to the physical characteristics of the strap (less elastic, less breathable, wider) it is quite uncomfortable for anything else than pool swimming. The HRM-Tri is more comfortable, but is not optimal for pool swimming, because it is not chlorine-resistant and does not stay firmly on your body when swimming.
And the HRM-Tri has running dynamics, whereas the HRM-Swim does not.
You can't use "batch" mode with HRM-Swim or HRM-Tri and the va3, so it only works like a regular HRM belt.
For watches that do support the batch mode, the way they work is they do not transmit HR data in the water, but send it in a batch when you get out of the water. This requires extra "smarts" on the watch. This is a function mainly on tri/multi-sport watches with Open Water Swimming. The fr 645 doesn't support batch mode either.
the va3 also doesn't support the RD pod or Running dynamics (the Garmin running Power DFs aren't supported on the va3), but that's something that is supported by the fr 645
There's an app in the IQ Store called Outdoor Sports+ that will track open water swimming with GPS and HRM. It works well - I used it a few times on a recent trip to Cuba and it seemed quite accurate!
