Echo City Team Up #2: Under the Sea

From publisher blurb: When Wave Master learns that the Horn of Poseidon is up for grabs, it falls to Meridia and Silverfish to stop him in this undersea adventure. This pay-what-you-want download is an adventure supplement for the Sentinels of Echo City superhero RPG. Although you need those rules to play this adventure, the content is easily customizable to your superhero game of choice... which SHOULD be Sentinels of Echo City now that I think about it, so then you're all set.