Piranha

Two teenagers come upon an apparently abandoned military installation at night. They take advantage of what appears to be a swimming pool to skinny dip. The teenagers are attacked by an unseen force in the pool and disappear under the water. A skiptracer named Maggie McKeown is dispatched to find the missing teenagers near Lost River Lake. She hires surly backwoods drunkard Paul Grogan to serve as her guide. They come upon the abandoned compound, which functioned as a fish hatchery. They discover bizarre specimens in jars and indications of an occupant.