Prisoners Of The Lost Universe/The Beach Girls And The Monster

In Prisoners of the Lost Universe (1983), three people are transported into a parallel reality, where they find they must use modern technology, but medieval weapons, in order to save the citizenry from a murderous warlord. In The Beach Girls and the Monster (1965), Surfers are being brutally murdered. Is the culprit a sea monster or just one of the teens jealous parents?