Logging this here as a good idea for bigger groups: Temporarily share your location via Whatsapp.
I guess this is probably a good idea as everyone will have Whatsapp installed and know how to use it, and it’s pretty reliable. Maybe as a coordination more than pure safety, but never hurts
I’ve had pretty good luck sharing my positon in Google Maps with my shuttle driver. The main exit point and alternates are saved in a shared list in Gmaps so it is super easy for her to see how my progress is going, and whether I am heading to the main pickup or bailing out to an alternate.
Any device loses GPS fix at least briefly when you crash. Different apps handle this differently, Gmaps does report some pretty wild position deviations sometimes until a good GPS fix is regained, which is my main complaint with it. Will have to test Whatsapp to see if it smoothes things any better.
Interesting about Google Maps, I’d presume it kills battery constantly trying to reroute you to a road? It’s surely not designed for how you’re using it?
It’s fine, it just shows a pin at your position on the water or off
While you still need a phone - I use the “safety and tracking“ feature on the garmin app. The watch gps does a great job at being accurate GPS and it even broadcasts heart rate data so your friends can see how pinned you are 
As @J_L says no need to have a route running, just have to make sure battery optimisations are turned off in Android so it doesn’t get killed if it ends up running in the background. Uses a bit of battery, but only a few % over baseline.