The Mayhem of Mermaids (The Adventures of Smith and Jones #2)

Vampires, Mermaids, and Tea!
Dr. Nettie Jones is an agent for The Society for the Exploration of the Unexplainable, a secret organization protecting England from various sources. She recently helped defend Mother England, and possibly the world, from an alien invasion.
And she's half-vampire on her mother's side.
While working in the London Society headquarters, Nettie becomes caught up in a new conspiracy--one coming from the ocean itself. Tracing the attacks to a small area in northern Wales, Nettie and the rest of the London contingent troop north to try and stop it before things get worse.
Fairy tales tell of loving mermaids wishing to come to the land. The truth is far more brutal, and the mer-people want far more than just to come on land for a visit.