Now, I've seen quite some rubber-suit monster, creature from wherever and suchlike, but man, this one really is boring. It even succeeds in being so bad that it's "good" which makes most of these movies fun to watch. But "The Terror beneath the Sea" lacks any suspense, storytelling devices or actors-not-being-a-cardboard figure.
The story is rather cheap, maybe even for a flick like this one, but hey, this would have not made the movie bad as such most old Sci-Fi flicks and films like this have a very bad narrative and still are entertaining. The "story" of this movie has been written down by some other guys here all over the page, so I won't sum it up again, but rather question some points
Apart from dragging on without building up any suspense or anything story-like beyond "now this is a master plan to conquer the world that did not work in some other 30+ movies they surely botch this one", the characters are all totally stupid (Chiba's Ken being the only one hardly managing to be likable). Like the bad guy, I forgot his name, is very evil indeed. Not only does he wear menacing sun-glasses he also sneers all the time, gives us some throaty laughter and comes over with the usual wanna-be World Dominator one-liners. I mean come on, sun-glasses in an underwater city? Where they afraid to make the movie to cheese if they would have given him a cool mask like Dr. Doom from 10,000 fathoms? Anyway, he is a total nut-case in fact, he never does anything but point out his glorious plan, while the work is done by his doctors-turned-henchmen. In the end he can't control his own "Water-Cyborgs" (bad looking rip-off's from the Creature from the Black Lagoon) and makes a run for it. Of course he is stopped by Chiba's hero, but only after Mr. Villain fails to shoot him at point blank range.
One thing I found quiet amusing was the shrieking and whining of Peggy Neal's Jenny, who is so occupied with her looks (they get a little bit mutated themselves), that if she survives the movie, she will surely commit suicide when she gets some wrinkles from age.
Generally, what is this thing with the mutations anyway? The "stop-motion" scenes with some other guy being transformed into a hideous Water-Cyborg (means looking ridiculous and being controlled by a WorkFightStop dial in the villain's HQ) looks like they did put butter or curd onto the poor guy to simulate a "mutated skin". Then they went into an aliment-frenzy and threw all other stuff onto him all which looks like some sort of milk produce. At last they put on some chips (posing as scales) and, hey, here's your average fish soldier.
There are also some guys from the US Navy, who are first reluctant to do anything, then see their wrongs and are over-anxious to to something and then come up with doing nothing more than blasting the villain's underwater city to kingdom come. They by the way overact so completely, and are so badly dubbed that it hurts but, as I said, they don't do much for the sake of the narrative.
Overall, what could have been a standard Sci-Fi fun flick with some silly fun is sadly completely sub-standard and just rolls along rather drowsy. And the ending scene is so completely terrible "a little laugh at the end" stuff with yet another attempt to break the sonic barrier of cheesiness that you are really happy that this has finally dragged itself to it's end.
Stay with "The Creature from the Black Lagoon" or "The Beast from 20,000 Fathoms" if you want some underwater monsters with style.
Review by Great-Cthulhu from the Internet Movie Database.