The Bowels of the Sinai
An underwater treasure locale far from shore, with three targets.
Get the highest one first, in the shell of an old boat.
Go up for breath, then dive back for the second, very nearby, below a rocky over hang.
Grab the treasure and, in that same tunnel, look for the narrow passage leading north.
If you get turned around, remember that the passage is north, and the fallen boat wreckage should be on your right.
Follow the passage to the white chest containing the last treasure, and remember you can fast travel underwater.