Trade Federation submarine

The Trade Federation Submarine (more commonly known as the Trade Federation U-boat)was one of the very few dedicated submarines to see action in the Clone Wars. Unable to be detected from the air until surfaced, it was perfect for ambushing seaborne convoys or defence of islands or costal regions. However, it had to be surfaced to use its weapons and when surfaced was vulnerable to air attack. The missiles could only target sea or land targets, but the laser cannon could target air units. Having no shields, the U-boat mainly relied on its hull armour for protection, but a determined strafing run by aircraft could still destroy the U boat in one pass. When an attack was detected, the U-boat usually attempted a crash dive, as laser or blaster bolts could not harm it underwater.

Sources: Star Wars: Jedi Starfighter