From the handbook (Page 9):
Built by Navy Yard.
Mobile unit - Ship - Weapon: Mortar.
A small pedal-powered craft that travels just under the surface - The Submersible is still visible to the enemy and susceptible to fire. Lets loose ye olde Mortar.
Note: The Submersibles can only attack non-flying units and they cannot transport any troops at all. They are actually aquatic catapults, so they can attack above obstacles, like walls or cliffs, but better be used against stationary targets only.