After some frustrations in tracking my open water swims and races with the fēnix 5, I decided to do something a little wild. I'd read around online that some people have experimented with taking their vívoactive HRs and clipping them to their goggles, or in their swim caps, and getting decent results.
I hadn't seen any experimental data or comparisons posted by anyone anywhere, so I figured it was worth a shot. Despite looking like a doofus during my last open water race, I donned my vívoactive HR to my goggle strap, strapped the fēnix 5 to my wrist, and pitted them both head-to-head.
Starting the workout on the vívoactive HR takes some finesse since you'll need to do it without looking at the device. Alternatively, you can start the workout before strapping it to the goggle strap and then rush to attach it before the race starts, which I decided was a little too much work (didn't want to be fiddling with my goggles within seconds of the start). I started workouts for both trackers within a few seconds of each other, and ended both at around the same time as well.
I'm not going to ramble on here; the results speak for themselves. Just check out the images below (don't judge, I'm not the fastest swimmer, and my sighting could use some work :) ).
The vívoactive HR is light-years more accurate, and gives almost per-second updates on speed and location, whereas the fēnix 5 only provides occasional GPS updates. If you want the source data, the images are hyperlinked and will take you straight to the workouts on TrainingPeaks. The fēnix 5 does capture stroke data—if that's an important factor to you—but otherwise falls short.
The reason I found this surprising isn't that it worked the way it did; that the vívoactive HR won out makes sense since the GPS sensor is exposed to open air for the vast majority of time while it's clipped to the goggle strap, and almost never fully submerges. The fēnix 5 has to re-lock GPS within a span of no more than 1.5-2s between strokes. One of the two devices clearly has the upper hand here.
The surprise is that not more people are doing this, or have figured out a way to make it less nerdy. Perhaps a small device that you just clip to your goggles would make for a decent product. Or, if there was an adapter to the vívoactive HR that allowed you to remove the straps and lock it onto your goggles, that wouldn't be a bad solution either.
In the meantime, I'm just going to keep using my fēnix 5 for races and keep the vívoactive HR in the swim bag. It's a little bit of a hassle to get the workouts started on the vívoactive HR, and not worth the extra stress, but if you do want a solid map of where you swam in open water, it's a solid way to go.