When a Storm strikes, look at the top card on the Draw Pile. The direction of the storm is indicated by the color of the storm symbol. It's either blue, red or purple.
The Blue Storm means the top board is rotated clockwise one quarter revolution, Red Storm is counter-clockwise. A Purple Storm means half a revolution.
These directions are also indicated by the colors of the arrows on the board.
When rotating the board, make sure to realign the boats in the corners.