Walkthrough:Treasure in the Depths
"Treasure in the Depths" is a job in "Tide of Terror" of Sly Cooper and the Thievius Raccoonus.
Walkthrough
As explained, your task here is to destroy a total of forty treasure chests using the submarine's cannon, before you can take the treasure key. Follow Bentley's instructions by first moving the sub around with 
and then firing its cannon using 
. A common tip with these types of vehicles is that you can always move and shoot in different directions.
Raleigh's gang of crabs will attempt to stop you from destroying the chests. The number one rule here is to not let any crab take a chest into a tube, or the mission is a failure. Do whatever it takes to stop them. As the mission progresses, the chests will begin dropping more frequently, and the crabs will appear more frequently as a result. The best strategy is to just keep moving and firing.
Note that if any crab is killed right beside a tube while carrying a treasure chest, the chest will disappear, but extra chests will always drop down.