Skylanders Spyro's Adventure soul gem locations guide

Gem #6: Oilspill Island

Unlocks: Boomer’s “Troll Bomb Boot” Ability

Proceed through the level until you reach this set of crystals.

Head left, smash the wooden gate, and head down the steps to the south to reach the water.

Using Gill Grunt, follow the pipe and swim clockwise to find a Soul Gem.