Three-Barrel Cove (heroic)/Elusive Bounty

First up: This is a long swim in an entirely flooded passage. Make sure you have an a Underwater Action item, or at least a 3-minute duration Water Breathing Potion, before attempting it.

Go to the spot on the map designated. If you haven't mastered the art of jumping and casting from the water, the Rotten Shark is the nearest shallow land/water where a spell can be cast. Swim down as far as you can. Then look for a vertical opening at one of the "clock positions of 3, 6, 9, 12" (it may help to instead think of these as the cardinal directions.) It allows you to swim horizontally further into the caves. When you can swim no further, swim up in that alcove as high as you can go. The rocks form a "T" and at the center of the t's crosspieces is a small opening in the rock to reach the bounty. Have several inventory slots free before even diving to collect all the loot.