From the Super Mario Wiki
“Go home and cry, babeh!”
Naval Piranha is a recurring Piranha Plant boss. She started out as a normal Piranha Plant, but was altered by Kamek's magic during the events of Super Mario World 2: Yoshi's Island. She grew to gigantic proportions, with long thorny stalk and two secondary Naval Buds. As her name suggests, Naval Piranha inhabits the water. In battle, she spits out Nipper Plants; it is unknown if they could grow into large, water-dwelling plants like Naval Piranha.
Super Mario World 2: Yoshi's Island
Naval Piranha is the boss of World 3 in Super Mario World 2: Yoshi's Island, and reprises the role later in the GBA remake. After being magically enlarged and enhanced, she battles the Blue Yoshi, who throw eggs at her only weak point which is her large and bandaged belly button (with the "X" band-aid). After being hit with enough eggs, Naval Piranha is defeated.
A famous cheat, exclusive to only this boss, is that Yoshi can beat her before the fight even starts. If Yoshi stands on the very edge of the ledge Naval Piranha is rooted to (or jump from the water and aim) without triggering the scene, he can simply throw an egg at her "unmodified" form to defeat her in one hit. A surprised Kamek then approaches, shouts "OH, MY!!!" and flies away, thus Yoshi clears World 3.
Yoshi's Woolly World
Naval Piranha reprises the role of a boss, in this case being the boss of World 4. After being enlarged by Kamek's magic, the monstrous plant jumps to the foreground inside the pipe, and calls forth its Naval Buds. Shy Guys will then throw bombs at the foreground, which Yoshi must swallow and spit it at the mutant flower. It will explode in its inside revealing its weak spot: its belly button. Yoshi must ground-pound it. Then, the Naval Buds will create a series of attacks that Yoshi must avoid. Repeating the process two more times will end the boss battle.
Mario Party Advance
Naval Piranha makes a reappearance in Mario Party Advance. At the Bowser Lab, choosing to see the "scary monster" unveils Naval Piranha. Afterward, Bowser has the player face her in the minigame Koopa Kurl. After winning, the player gets to keep the minigame. The end of game credits say she now lives and thrives in the desert.
Like Birdo and Vivian, Naval Piranha has had some controversy surrounding her gender in the past. In the original Super Mario World 2: Yoshi's Island, Naval Piranha's gender was not mentioned, and since there was no visible evidence that she was a female, most players referred to Naval Piranha as a "he". Later appearances, however, have stated that Naval Piranha's gender is female. While this may sound strange at first, some species do have separate male and female plants; these are known as dioecious plants. Other Mario plant individuals with definite genders include Petey Piranha and Hayzee.
Names in other languages