Instead of something complicated what about something simple.

Take the principle of dipping sonar where they dip the head into the water
from a helicopter to ping.

Invert it

So now you take a gps antenna with ground plane and put it onto a float.
I'm assuming this is clear water thats too busy to safely surface?
A hundred feet of cable put the float on top for almost all sport divers
A few seconds on the right gps will give you a course and distance
back to the origin.

Wind the cable and float back down or let it stream but it should allow you
to find the way back to the safe area. If you had a full face mask you
could even
rig in that radio...

My 2 cents

Dave

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