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 briefing Sly on the mission[src]
Sly hopped onto a swamp skiff and set out into the lake filled with piranhas. These fish were the only source of oil for the skiff's flamethrower. Using the fuel provided by the fish, Sly was able to light the 25 tiki torches within two minutes, allowing him to unlock the gate leading to the treasure key.