The Ocean Treasure is a one-time salvage item in Endless Ocean 2.
This is found in the Treasure Room of the Cavern of the Gods using the clues in Okeanos Tablet 5. It is a mysterious chest, and CAN be found before the end of the main plot, but only if you complete the Cavern of the Gods map first. While you have the ten minuets to escape, turn around and use the multisensor to find the treasure. Have Nancy appraise it when you get back and you will be 65,000 pelagos richer. This is the most expensive treasure in the game.
The appraisal reward is 65,000 pelagos.