Because the budget was so low, Ray Harryhausen saved money by building his octopus model with six rather than the correct eight tentacles. He tried to pose the creature so this lack of the right number of arms wasn't apparent.
Several subs appear in stock footage. The jet-propelled torpedo gets loaded onto a real sub that appears to have a fake conning tower, probably built over the real conning tower to make this sub visually match the sub that appears later. It's hard to read the sub's number in the torpedo-load scene, but it appears to be 334 --USS Cabezon. The Cabezon arrived in California in 1953 to join the reserve fleet and might have been undergoing inactivation when the film crews set up.
City officials refused to allow the filmmakers to shoot on the real Golden Gate Bridge, because they didn't want the public to think that the bridge could actually fall. Ray Harryhausen recreated the entire bridge in miniature.
When the Navy first uses depth charges on the octopus, the destroyer most prominently featured is DD-540, which is the USS Twining. Launched in 1943 the Twining won many battle stars for action in World War II and in Korea. She was sold to the Republic of China in 1971 and stricken in 1999.