Kirby can turn into a cute little submersible when he has to work underwater. You can guide his missiles with a rainbow rope to hit hard-to-reach targets. But that periscope is just for show—Kirby doesn't even know how to use it!”
When in this form, Kirby takes on the appearance of a submarine—he retains his pink body and face, but he gains a yellow rudder, a red marking on each side of him, probably meant to represent a submarine window, and a pink periscope. The torpedoes that he fires are white and red.
Kirby can transform into Kirby Submarine in the stages Kirby Submarine's Torpedo Time and Deep-Divin' Kirby Submarine. While he is in this form, Kirby must remain submerged at almost all times. When the player taps an area on the touch screen, Kirby Submarine will move to that position. The Kirby Submarine can only fire torpedoes automatically in a straight line, but rainbow ropes can help to guide them in a different direction. If Kirby collects 100 Point Stars, he will become able to shoot a single super torpedo that is more powerful than the regular ones. These super torpedoes are also more durable, and they do not explode immediately when they hit a surface; instead, they bounce around for a while until they go off-screen or until they simply disappear. They are able to break through Metal Blocks.
Tackle underwater foes with torpedoes.”
— Official Kirby and the Rainbow Curse website
When Kirby is transformed into Kirby Submarine, Elline momentarily transforms him into Kine the Fish.
Notes from the ESRB that explain Kirby and the Rainbow Curse’s E rating explicitly mention Kirby Submarine, saying: "As players traverse side-scrolling levels, they can transform into various vehicles (e.g., tanks, submarines) that fire projectiles at enemy creatures."