In reply to girlymonkey:
> However, when I swim (almost always outdoors) it really struggles with getting and keeping a GPS signal. The tracking shows all sorts of weirdness. I just stick my phone in my tow float and record swims on that, but don't know what you would do in the pool.
Yes, I don't think GPS signal transmits very well underwater, so when you are swimming the signal is lost everytime your arm goes under the water, and there is limited time to re-connect when your arm is out of the water. Some of the newer/higher end Garmins have some funky technology which does a better job for open water swimming (dual band GPS and some caching technology, I think) and are much better than older models, but the tracks are still usually worse than for running/cycling etc.
Indoor swimming is actually simpler, if you choose a watch that has a pool swim function. Of course, you don't get a GPS signal at all in an indoor pool. These work by having a motion detecter that is trained for swimming - effectively, they can tell when you are swimming smoothly, when you stop, and when you turn. You tell the watch how long your pool is, and it just counts lengths, adds them up for you, and provides splits. My slightly older Fenix 5 works very well for this as long as I make a nice definitive kick off the wall at the end of the pool (doesn't have to be a tumble-turn). I don't know off-hand if either of the Forerunner models you mention have this functionality.
DC Rainmaker is a great source for in-depth reviews of sports watches and fitness tracking devices: https://www.dcrainmaker.com/
Post edited at 12:26