From the walkthrough:
Be sure that you have scuba gear, metal detector and shovel.
Sail SE of the island. See a broken circular coral reef. Enter the circle of rocks.Save game here. Double check that the air tank is full - 100%.
Read El Toro's journal concerning the Caridad.
Dive down by clicking on the rope ladder left of boat.
Automatically be in scuba gear holding the metal detector.
At top of the screen, the blue line is the air tank amount and the red line is the pressure underwater. Watch those 2 lines.
See brain coral growing just beside the shipwreck.
Swim to bottom right until the shipwreck and then over the middle part of the deck.
Once the metal detector reads full bar, right click and automatically use the shovel.
See the chest with the insignia and name Caridad. Remember this spot.
Open the chest and look to study the lock.
