You can see how this could be pretty offensive, and you know, it sort of is. However, the fact that the creatures are guys in rubber fishman suits with large rubber brains sort of mitigates against the offensiveness. To put it bluntly, a man raping a woman is pretty shocking (and something I've complained about with some of the other exploitation films I've talked about here) but a rubber fishman with a huge brain raping a woman is a bit more silly than offensive.
The fishmen are actually genetic mutants created by a corporate cannery that has injected the salmon in a small fishing town with chemicals in order to make them breed faster. There's local intrigue because the fishermen want the company in town with their fancy genetically-modified salmon, while the local Indian tribe wants them to stay away from the traditional lands, and doesn't trust the company in general, what with their crazy chemicals. These are all B-movie characters, so you have the classic conflict- Indians love the land and rednecks love beating up Indians. Well, of course, the Indians are right here because those genetically-modified fish are just raping and killing up a storm.
Our hero is Jim Hill (played by B-Movie stalwart Doug McClure, who you might remember from such films as...) who thinks there's something behind the recent string of dog-killings. His wife, Carol (Cindy Weintraub) agrees and gets some screen time as the damsel in distress, but is entirely too hot not to have screen time as the damsel in the shower. The local loudmouth bigot is Hank Slattery (Vic Morrow, a TV stalwart who was killed a few years later on the set of Twilight Zone: The Movie), who starts a brawl at the annual Salmon Festival with the Indian hero, Johnny Eagle (Anthony Pena), who is pretty much like every movie Indian in that he has a deep relationship with the earth and gets beat up by rednecks. There's also a corrupt mayor and a hot lady scientist who created the fishmen with her crazy chemicals.
And, of course, there are also a good number of horny teenagers coming to the beach to have sex, but ultimately getting fish-fucked or fish-massacred, including a young man who actually seduces a sexy girl via ventriloquist dummy, a likely first in cinema, or reality. Finally, there's a victimized girl in the hospital impregnated with a fishbaby.
The chum hits the fan at the yearly Salmon Festival when the Humanoids decide to go a rampaging and even Miss Salmon gets her top ripped off. The massacre scenes are remarkably bloody and the Humanoid suits, by Rob Bottin, are fairly awesome. The film also works because it's outrageous without ever veering into intentional satire. Also, by this point, Roger Corman pretty much knew what his audience wanted: monsters, tits and gore.
Extra: Believe it or not, punk singer Jello Biafra wrote a song about Humanoids from the Deep: