The Gungan military launching ship was a long narrow submersible that could navigate the underwater caverns deep in the Naboo seas. They were developed after the Gungan's war with the Trade Federation as a quick way to transport troops.
The submersible was long and thin. The skin of the outer hull was made out of a blue mineral that helped to camouflage them in the water. They carried numerous warriors.
They were used to surprise an imperial battalion when Ferus Olin and Solace led a combined Gungan/Naboo militia against Malorum and his forces. The surprise attack by the Gungans lead to a complete routing of the imperial battalion.
The Last of the Jedi: Death on Naboo (First appearance)