Attack of the Crab Monsters -- Roger Corman
Seven years before getting stranded on Gilligan's Island, the Professor (Russell Johnson) was stranded on another island with giant crab monsters. It's not the same character, but he is tasked with building a radio in Crab Monsters, so he may as well be.
I enjoy these cheesy sci-fi flicks from the '50s. They fun and entertaining, they don't waste a lot of time (this is just over 60 minutes), feature lots of amusingly casual sexism no matter how smart or authoritative the woman is (and there is usually one woman), and take the subject seriously. The latter part is especially welcome given that we live in the Irony Age.
Much like Godzilla, Rodan, Them!, and several dozen others, the crab monsters are created in the wake of nuclear testing. A crew of scientists are sent to the island to investigate the effects of the radiation and are picked off one by one. What's great about Crab Monsters is that it's suitably nuts. After the crabs eat someone, they absorb that persons brain and can speak to the humans through metal devices such as a gun or the morse code lever-thing. Not only that, but knives and other weapons do know damage to the crab monsters and actually pass right through them because the crabs are made of negative energy. It's awesome.
There are all the normal pitfalls of cheesy sci-fi like bad exposition and clearly fake monsters (terrific fake monsters. I want one for my yard), but you don't watch movies like this without that in mind going into it. It's just fun times.