Age of Atlantis by Jo Whittemore (CW’s Supergirl #1)
Supergirl (aka Kara Danvers) has been running across average citizens performing amazing feats all over National City. But that’s not even the weirdest thing going on. The Department of Extranormal Operations has captured a mysterious humanoid sea creature. Supergirl must find out what has drawn him to National City, and his connection to this surge of super-citizens.