You and Brink go to the museum. Brink will help you to open the weakened door. And pick up some life crystals. Go to the water. The Monster eat turtle again.
Brink run away.
In order to go to the underwater cave, you must kill the monster. You find you can rebuild the turtle. Attached to the head is a small, thin neck bone with the thinner end attaching to the head. The second and third set of arms come immediately afterward, moving downward in size. Put the canister into the turtle, and apply the life crystal.