The ominous looking tool pictured above is a modern version of a grapnel or grappling hook. It has a certain menace about it, even though it's original purpose aboard ship was relatively mundane. As a grapnel, the thing is a form of kedge or anchor for small boats such as a launch. With it's backward pointed spikes, it is easy to imagine how the grapnel would catch on the sea floor or some other likely object (a rock, seaweed, etc.) and hold a vessel in place. A ring at the end of the grapnel would hold a rope, cable or chain and your launch would be quite secure until it was time to move on.
As early as the time of Hannibal and the Carthaginian wars with Rome, navies and pirates began using the grapnel for another purpose. As a grappling (or grapling) hook, the devise was used to "grapple" an enemy ship, bringing them close enough for boarding. In this capacity, the hooks were given spikes and the rope was shortened and attached to the yard-arms of the attacking ship. Sometimes, small grapnels could be shot from guns.
The grapnel was also used to hook a disabled ship for the purpose of towing her. This was particularly useful in the case of a ship on fire, where a very long cable could be utilized to keep the rescuing ship out of harm's way.
Finally, in a pinch a grappling hook could be used as a weapon. As witnessed on the Somali Pirate vs Medellin Cartel version of Spike TV's Deadliest Warrior, the grappling hook, though indelicate at best and certainly unwieldy, can cut up a pig carcass something awful. I'll stick to my cutlass, though, and save the grappling hook for boardings.