What scuba diving knife expands and blows up stuff?

Answer 1

There is probably a James Bond device that does this but I think you might be referring to something called an underwater shotgun. This is a device that was initially developed to enable people like downed airmen to defend themselves from shark attack but went on to be used by certain spear-fishermen in the late 1960s to kill sharks and rays. They are well illustrated in the book 'the shark hunters'. The device is basically a spear with a large bore shotgun cartridge on the end. The firing mechanism is a trigger on the spear tip. The cartridge was fired when it was pushed onto the prey. I would imagine you would want the firing bit as far away from you as possible, so the spear arrangement would make a lot more sense than a knife.

Fortunately this type of spear fishing is not very popular now, which is a really good thing. Diving is pretty safe sport when you consider the difficult environment, so taking firearms with you is not the most sensible idea, let alone the unsporting outcomes for marine wildlife.