There’s quite a bit of discussion of release mechanisms in this thread, so I’d suggest looking there first.
One of the comments there is:
It may also be possible to get/construct a linear actuator that can retract from a hole to release, or use an actuated razor blade to cut the loop the weight is tied on with.
This isn’t something I’m experienced with, but I imagine the transmitter hardware itself should be enclosed so it doesn’t need to handle the pressure, with an antenna sticking out to ensure the signal can transmit effectively.
