Sleep with the Fishes

From publisher blurb: The merman mafia had been content to operate out of the safety of Stark Lake, dabbling in minor crime in Stark City. But no more! The Atlantean gangster Krachla has captured a lakeside neighborhood in a forcefield, flooded it and mutated the residents into water-breathers (who he is holding hostage to ensure the non-interfence of the authorities.) He plans on ruling this sub-aquatic neighborhood as sovereign nation with himself as dictator for life. The police and army are powerless to stop him -- Can your heroes save the day from this Aquatic Al Capone?