Instructions
Complete the Knothole Island's "Huge Heat" quest then return to Knothole Island, speak to the chieftain and accept the "Drowning" quest. He mentions that there is a dig spot where the Storm Key is located underground.
Follow the golden bread crumb trail to a cliff. Dive off the cliff into the water then swim toward the footpath ahead of you.
Follow the path as it winds up a hill until your reach the specified dig spot. Use your spade to dig up the ground to find the Storm Key.
Run back to the water and swim straight across to another footpath. Continue to follow the golden trail until you meet a group of Balvarines. Continue along the trail through a village then into the lake once you defeat the Balvarines.
Swim straight ahead until you reach land. Continue along the trail until you meet a Hobbe ambush. Continue forward once the Hobbes are defeated.
Swim through a narrow lake to reach the Storm Shrine. Approach the door and unlock it with the key.