The Black (1970)

An unspeakable ancient evil pervades the water around Dante's Island once every 30 years. Six friends, home from college, learn of the phenomena's imminent return and decide to investigate. They ignore the dire warnings of the only known survivor of the black and set out to the island. Once there they find themselves trapped with two mysterious strangers who claim to be scientists doing their own research. As the body count rises, the only thing they can count on is death.