1/15 - In the first area you enter, head straight towards the large rock at the opposite side. The catfish is in front of the left side of this rock, above the connecting smaller rock.
2/15 - From the first catfish, look to the right and swim up between the large green pipes. The catfish is to the right outside of this broken window.
3/15 - After turning the wheel, head out the broken window. From these bubbles head right and along the structure towards some more bubbles in the distance. The catfish is to the right of these bubbles.
4/15 - Head back to the bubbles outside of the broken window. With your back to the window, look to the left of the two large pipes in front of you to see more bubbles next to a shipping container. Go towards these and kill the enemy that bursts out. The catfish is above the left side of the container.
5/15 - From the last catfish, look at the large structure to your left and you should see the next catfish directly to the left of that.
6/15 - Now look towards the large green pipes to the right of the shipping container. Swim up to find the catfish above the furthest pipe.
7/15 - Now go below them and face the large grate with the glowing spot to plant one of the bombs. Follow the pipe around to the left of the grate and when you reach the first set of bubbles, you should see a floating cube a bit in the distance. The catfish is between you and the cube, to the right of the car.
8/15 - Plant the bomb and continue along the pipe to reach the next wheel to turn. Take care of the two enemies then head towards where they came from (underneath large arch to the left of wheel). Activate Duke Vision to be able to see and you should be looking at a blue car with its headlights shining on a corpse. Swim down there and you should be able to see the catfish to the right of the corpse. (It’s easy to spot if you keep the corpse to the left side of the screen. Look at the plants to the right of the corpse and you’ll see the catfish on the right side of the screen.)
9/15 - Keep swimming in that direction to the nearest set of bubbles. In front of you will be a car with the catfish to the left of it.
10/15 - Follow the pipe to the next set of bubbles. The catfish is above the boat to the left, near the back end.
11/15 - Continue following the pipe and at the end swim past the dead EDF soldier to the next wheel to turn. Facing the wheel, look up to see the catfish.
12/15 - After turning the wheel and defeating the enemies, follow the next pipe. After reaching the second set of bubbles (the first being right next to the wheel), look towards the arrow on the pipe ahead and you should be able to see the catfish slightly left of the pipe.
13/15 - Continue along the pipe to the next set of bubbles. Once there, turn 90 degrees to the left. Swim up to find the catfish at the top of the rocks, slightly right, near some plants.
14/15 - Return to the pipe and follow it to the next wheel. Turn the wheel, then go past it and turn right to see an EDF ammo box with the catfish above it.
15/15 - Continue along the pipe on the right side of this large area. Once you reach the large wall, you’ll see an area to your right. Head over there and between the two large pipes coming out of the wall is the final catfish.
Credit goes to Ethigy on TA