Mysterious Island (1961) on Netflix

In 1865, during the siege of Richmond, Virginia, Union soldier POWs Captain Harding, Neb and Herbert escape in a balloon during a hurricane with two confederate prisoners, Sergeant Pencroft and journalist Spilett. The storm takes the balloon to a mysterious island near New Zealand in the South Pacific. Captain Harding proclaims himself leader of the group and they look for food. They discover that they are stranded on an island and soon are attacked by a giant crab that becomes their first meal. In the following days they build themselves shelter and find the island inhabited by other giant animals. One day they find two castaways on the beach: the aristocratic Lady Mary Fairchild and her sexy niece Elena. Later they find a trunk with weapons and a sextant and then they find the hut and journal of a man once marooned on the island by pirates. When pirates later return to the island, the castaways are surprised to be helped by Captain Nemo of the legendary submarine Nautilus. (Nemo -- notorious for his war against slavery -- was supposedly sunk and gone missing eight years earlier.) Finally, when a volcano threatens to obliterate their island, all must plan an escape.