I've thrown this one out to the Internet hordes before, with no luck. Help me, Obi-wan K'trauma! You're my only hope.
I saw this movie on late-night T.V. around 1976, give or take a few years. It was probably on SAMMY TERRY on Channel Four in Indianapolis. (If you don't know SAMMY TERRY, he's worth a look. Check out the link -- it's even got Nixon!)
Anyway, I remember this movie in black-and-white, but we had a B&W T.V., so it may have been in color, no thanks to my cheapskate Dad.
The opening scene involves some unscrupulous capitalists dropping nuclear waste on an old shipwreck. Through the magic of double-exposure, we see the nuclear waste transform the dead pirates into gill-men.
The next scene I remember is the gill-men coming up on land and menacing some bathing beauties. The image that is seared into my pre-'tween memory is of a gill-man disemboweling a bathing beauty by gently stroking his claws along her abdomen, releasing a torrent of Hershey's Syrup. It was the most gore I'd seen at that point in my life. And it looked really fake.
The movie I'm thinking of is most definitely not THE HORROR OF PARTY BEACH, which is an awesome film that I recommend to everyone. Nor is it the Italian-made, ROGER CORMAN-produced THE ISLAND OF THE FISHMEN. I have seen these, and ruled them out.