You need fuel from the flame thrower and the only source of oil are those piranha you see swimming around. Run them down with the boat - one fish fuels one shot. Hurry, you only have two minutes to get all the torches lit!
— Bentley telling Sly how to obtain flame thrower fuel[src]
First, Sly auto-jumped into the Swamp Skiff, and got launched into the lake. The piranhas tried to swim away, so Sly should avoid that by chasing them. He needed to ram 25 of them using the incorporated flamethrower to light the torches, but the vehicle only held 5 piranhas worth of oil at a time. The limit time to light all the torches was 2 minutes . When the 25 torches were lit, the water went down and Sly could enter the back of the area, where the treasure key was standing.