I’ve just finished this level and I have to say the sea serpent is the hardest part of the game so far. I think the serpent is blind and relies on noise to find Laura , when you come out the door run as fast as you can to one of the levers nearest to you before the cut scene , I found the left one to be the easiest .Get so that the lever target is red, wait until the serpent is looking away from Laura and then quickly use the grapple to pull down the bell. Start shooting the bell , this drives the serpent crazy . You may have to do this a couple of times then the serpent smashes the bell. Quickly run , jump to the lever on the right and do the same.
If you feel you have done this without taking too much damage run back to Arthur and save the game.
Run jump to the pier and do the same to the two furthest away levers. ( Im playing the game on a PS2 so I don’t know if it’s the same on a computer but I found that I had to do the lever on the right first or the one on the left just did’nt work.)
When you do finally get it I advise you to go back to Arthur and save the game.