When initially found this item will certainly appear as a large iron barrel. It has a secret catch which opens a hatch in one end.

Inside are 10 levers:

The apparatus moves forward at a 3" speed, backwards at 6". 

The 2 pincers extend forward 4-ft and snap for 2-12 hit points damage each if they hit a creature — 25% chance, no reduction for armor, but Dexterity reduction applies. 

The device can operate in waters up to 900-ft deep. It can hold 2 human-sized persons and enough air to operate for 2-5 hours at maximum capacity. The apparatus is AC 0 and will take 100 hit points damage to cause a leak, 200 to stave in a side. 

When the device is fully operating the whole appears as something like a giant lobster.