1) In Sea Caves (not Bayjin) that door is opened by a key given to you by Marten at the same time he gives you the DD.2) To destroy the T'rang ship transporter, first shut off gas safety sw in corridor reached from mine tunnels, second get into room at end of mine tunnel corridors move power sw from + to -, last use pluse pack found in mine tunnel coooidors or given as last mission from Umpani to open door in mine tunnel area that will not open or hidden door across from Z'ant go to computer tell it to increase power this will not only shut down the ship transporter it will destroy the ship and amy chance of a T'rang - Umpami alliance.
3) The hook and line is used to get behind the door in one above from below explore the cave you'll find a place it can be used.