Treasure of El Patron — Gary Paulsen

Tag pointed his flashlight into the hole and peered inside. It was impossible. The inside was hollow, like an underwater cave. Something shiny lay near the opening and reflected the beam from his light. He reached inside and pulled it out.
 
A pewter spoon.
 
If he could have made a sound, he would have screamed with joy. He tucked the spoon in his vest pocket and reached into the hole again.
 
A sharp stab of pain shot through his left hand. Something had hold of the tip of his thumb and was trying to yank him into the hole!
 
Tag Jones knows that somewhere in the azure water and coral reef surrounding Bermuda lies a sunken ship full of treasure. El Patron sank in 1614, and Tag's father died in a diving accident while looking for it. Tag won't give up until he finds El Patron -- and he's not scared off by the local legend that says the ship is cursed. But when two tourists ask Tag and his friend Cowboy to retrieve some mysterious underwater parcels for them, the boys find themselves in dangerous water, way over their heads!