I do know that this Island saw distribution in the United States after being picked out by Roger Corman and his New World Pictures. IMDB.com reports that -- as happened with Corman's pick-ups from time-to-time -- the producer had some new footage shot and added to the film in order to properly 'Americanize' it for audiences, an effort that commonly included added some known marquee faces to the existing story with these added vignettes. In that respect, IMDB.com asserts that Mel Ferrer, Jim Alquist, Cameron Mitchell and a few others only appear in New World's version of the production.
Also, I think it bears mentioning that director Sergio Martino (who also contributed to the script) considers this film the second installment of what has been dubbed his 'Adventure Trilogy.' (This information appears on IMDB.com's Trivia Page.) His first -- 1978's Slave Of The Cannibal God -- is an (ahem) delightful piece of bloody shlock starring Ursula Andress -- more like 'Ursula Undress' for this picture as it's revered for her nude body -- along with the great Stacy Keach, Claudio Cassinelli, and Antonio Marsina. The final chapter -- 1979's The Great Alligator -- stars Barbara Bach (who appears in Fishmen as well) along with Mel Ferrer and Claudio Cassinelli.
Interestingly enough, Fishmen's script appears to have been original derived from a story credited to the great H.P. Lovecraft. Again, IMDB.com indicates that Lovecraft's "The Shadow Over Innsmouth" served as inspiration for the tale. Though I'm unfamiliar with the tale, various Google citations indicate that the novella does indeed busy itself with the exploits of some sea devils on the shores of a Massachusetts' coastal village. Here's the film's plot summary as provided by the good folks at IMDB.com:
"After their prison ship sinks in the Caribbean, a group of prisoners and a doctor wash ashore on a seemingly deserted island. They soon discover a strange couple, who invite them to stay at their house. While the prisoners try to plan an escape, the doctor does some investigating, and soon finds out just what the pair are really doing, and why the prisoners keep disappearing mysteriously."
If that isn't enough proper fodder to interest you souls in venturing into this bizarre exploit, then how about the charm of a lovely lady? The lovely lady -- who went on to be a 'Bond Girl' for Roger Moore in 1977's The Spy Who Loved Me -- got her start in the role of 'Nausicaa' for an Italian television miniseries adaptation of Homer's "The Odyssey" ("Odissea") and continued to find work in projects afterwards. If her pretty face -- alongside these gruesome sea creatures -- isn't enough reason to enjoy this production, then I'll leave you to your pursuits.
As always, thanks for reading ... and live long and prosper!