Be sure that:
- Read El Toro's journal concerning the Caridad.
- you have scuba gear, metal detector and shovel.
- the air tank is full - 100%.
- 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.