Creatures from the Abyss (1994)

In the long-line of mutated nature-runs-amok flicks, here comes a true original - Plankton - a low-budget shocker starring, you guessed it, killer plankton! When a young couple comes across an abandoned boat, little do they know the looming crustacean danger that lies in their future. It seems that weird biological experiments were being conducted before the crew mysteriously vanished. Soon the horror ignites as the horny, brainless couple begins to truly grasp the terror that awaits them when monstrous killer plankton come calling. ~ Jeremy Wheeler, Rovi