The Ravening Deep. The Sanford Files, #1

A nightmarish power unleashed from the depths infiltrates Arkham in this nautical-horror pulp adventure from the acclaimed Arkham Horror game world When dissolute fisherman Abel Davenport discovers an ancient temple in the deep ocean, he under the influence of a long dead god. In his attempts to restore the god's cult, Abel unleashes a plague of twisted doppelgangers on Arkham. Horrified by the consequences, Davenport realizes that he alone cannot stop the monsters from resurrecting the Ancient One.
Sometimes the only way to end one cult is to start another. Teaming up with redeemed cultist Diana Stanley and notorious thief Ruby Standish is the first step. The second is convincing Carl Sanford, the powerful leader of Arkham's Silver Twilight Lodge, to join their cause. Together they might be the only hope of averting a cataclysmic eldritch invasion.