Start the Beach memory level. Get the map, burn the map on the fire, pick up the metal detector, find the two buried bracelets, put the bracelets on their respective pedestals. Enter the boat.
Okay, so now we're underwater! There will be a couple of whales swimming around. One of them will swim past you, and through an invisible wall. But there should be another whale further up. Swim up to it, and when you're close to it the achievement will unlock!