It can be built by placing down one Obsidian block on a Destroyer, under y 66. The Obsidian must be touching water outside the ship in order for it to launch.
Speed: 9 m/s
Place down a sign on the Obsidian block and type
DepthCharge on the first line.
After right clicking on the sign, the Depth Charge is launched in the direction that the sign is facing, and then drops in water. It will detonate in the water and deal damage to all Submarines in a 50 block radius around it.