You and Brink go to the museum. Brink will help you to open the weakened door. Pick up the canister on the floor, and pick up some life crystals, too. Go to the water. The Monster eats another turtle.
Brink runs away.
In order to go to the underwater cave, you must kill the monster. You examine the loose bones and 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. If you've done it right, you can put the canister into the turtle and apply the life crystal.