The Coven of Frostfall Bay

From the introduction:

The environment surrounding this dungeon is a harsh wintry climate. The capsized longboats and skiffs that comprise the hag coven’s den are tucked away in a cove that rests near northern trade routes. The hags have been using their combined magics to lure ships close to their cove and smash them against the icebergs which float within their lair. As the sailors tumble into the frigid waters, their true horror begins as some are devoured by the giant eels while others are pulled in to the curs-ed frozen depths to drown and become the undead servants of the hags.