Had my Vivoactive 3 since end of October and I’m really happy with it, it’s barely left my wrist since it arrived.
For swimming it measures lengths, gives calcs for strokes per length/per minute, stuff like that, but isn’t set up for open water swimming.
GPS works well for walking, cycling (& I guess running but not tried that) plus it has loads of downloadable apps available as well.
The only thing I’ve found a bit iffy sometimes is the level counts going up stairs – usually works well but occasionally misses a flight.
Battery life is really good, without using GPS I’ve got 8 days between charges and it normally takes less than an hour to get back up to full power.
Controls are simple and the ‘find my phone’ feature has been a lifesaver on more than one occasion.
It also takes a standard 20mm watch strap which a lot of smart watches don’t, so if there are any issues with it you can swap it out easily.