“Panic” is the very last thing you want or need in a survival situation. If someone has their hands around your throat and is holding you underwater, raise your arms above your head, clasp your hands together, and drive your clenched fists down between their arms. This will usually break their hold, enabling you to deliver a punch to some vital area… the groin, the solar plexus and the throat are good targets. The blow should be abrupt and have as much of your weight behing it as possible. Repeated blows in the same spot are sometimes more effectve. Continue to attack for as long as you can, until your opponent is disabled or until you have to catch your breath. The key is to attack, attack, attack.
If someone has their hands around your neck from behind, reach up with your free hand, grip a finger on the same side, and throw your arm sudeenly out to the side. This will often dislocate the finger, which is very painful. While the opponent is somewhat stunned from the dislocation or broken bone, whirl to the other side, smacking your open palm against their ear.
There are many, many techniques for surviving this sort of attack. far too many to go into here.