It Came from Beneath the Sea

It Came from Beneath the Sea is a 1955 monster movie featuring special effects by Ray Harryhausen.

You can watch it online at this link. Here's a trailer:

1000 Misspent Hours says the film "is a reasonably entertaining flick if you go in not asking too much of it" but adds that it "would be pretty damned forgettable if it weren’t for Ray Harryhausen, and it is from his colossal octopus that virtually all of the movie’s entertainment value derives." Moria loves Harryhausen's special effects but says, "The problem is that one has to plod through three-quarters of the film to get to the monster sequences."