Black Boat Units found in Dual Strike
Introduced as a Black Hole Invention in Advance Wars: Dual Strike, the Black Boat is capable of transporting a total of two Infantry or Mech Units across the sea. In addition to its transport capabilities, it is capable of supplying and repairing an adjacent unit by 1HP.
However, like many other transports the Black Boat does not have any weapons and cannot fire on any enemy units that choose to attack it. Plus, if it gets destroyed with units on board, those units will be lost.
After finding the hidden map in the mission Black Boats Ahoy! and completing the subsequent hidden mission The Long March, you'll be able to deploy your own Black Boats for 7,500G each during that Campaign, the Versus maps, War Room missions, and the Design Room for customizing your own Multiplayer battle.
Black Boats are great for support, as a timely Repair can make the difference between a unit's survival and a unit crashing/sinking due to running out of fuel. The extra HP also helps for attacking purposes.
As it is also a transport that cannot fire, Black Boats require support from other nearby units, mostly Air and Naval Units.