I saw this same idea posted 5 years ago (closed thread), but I think it still holds water..
Being a terrible swimmer, I can't do freestyle, I swim with 2 devices, one on a two float and one on my wrist (OWS with GPS off), and merge the two FIT files later.
A better solution would be, put memory in a GPS receiver, with a BLE/ANT+ chip, place receiver on a tow float or under swim cap or attach to goggles, after the swim download the track from the receiver to the watch, just as you do with a HRM-Tri to download heart rate.
www.garmin.com/.../ if you think this would be something you'd like.