Creatures of the Pool — Ramsey Campbell

Gavin Meadows always worried about his father. The old man could appear a bit paranoid at times, and his research projects sometimes seemed a little crazy. But now his father's missing, and Gavin is more than worried. And the more he looks into his father's research, the more frightened he becomes. This time his father might have really been on to something. Something truly terrifying. For the more Gavin learns of the secrets hidden beneath the city for centuries, the more he realizes the people he sees on the street every day may not be people after all.