From the Super Mario Wiki, the Mario encyclopedia
Naval Piranha is a recurring Piranha Plant boss. It 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. It grew to gigantic proportions, with long thorny stalk and two secondary Naval Buds. As its name suggests, Naval Piranha inhabits the water. In battle, it spits out Nipper Plants. Naval Piranha's name has a double meaning, referring both to its aquatic area and the large belly button it grows, i.e. its navel.
In the original strategy guide for the Super NES version of Super Mario World 2: Yoshi's Island, Naval Piranha is referred to as a male; however, in the guide for the Super NES Classic Edition, Naval Piranha is referred to as a female.
Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
Naval Piranha is the boss of World 3 in Super Mario World 2: Yoshi's Island, and reprises the role later in the GBA version. After being magically enlarged and enhanced, he battles the Blue Yoshi, who throw eggs at its only weak point which is its large and bandaged belly button (with the "X" band-aid). After being hit with enough eggs, Naval Piranha is defeated.
Exclusive to this boss is that if the player stands on the very edge of the ledge Naval Piranha is rooted to (or if the player jumps from the water and aims) without triggering the scene, the player can simply throw an egg at its "unmodified" form to defeat it in one hit. A surprised Kamek then approaches, shouts "OH, MY!!!" and flies away, and thus the player clears World 3 without fighting Naval Piranha.
The official English website for Super Mario Advance 3 misspells its name as "Naaval Piranha".
Yoshi's Island DS
During the introduction to Yoshi's Island DS, Naval Piranha makes an appearance alongside the other castle bosses in the game as the events of Super Mario World 2: Yoshi's Island are recapped.
Yoshi's Woolly World / Poochy & Yoshi's Woolly World
Naval Piranha reprises the role of a boss in Yoshi's Woolly World and Poochy & Yoshi's Woolly World, in this case being the boss of World 4. Not including Baby Bowser, Naval Piranha is one of the only two bosses from Super Mario World 2: Yoshi's Island that make an appearance in Yoshi's Woolly World, the other being Burt the Bashful. After being enlarged by Kamek's magic, it jumps to a pipe in the background, and calls forth its Naval Buds. Shy Guys will then throw bombs at the foreground; which Yoshi must swallow and spit out at the mutant flower's open mouth, causing it to swallow the bomb and reveal its weak spot: its belly button. Yoshi must ground-pound it to deal damage. 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.
Similar to Super Mario World 2: Yoshi's Island; Yoshi can throw a yarn ball at the sleeping Piranha Plant before triggering Kamek's appearance. This causes a slightly different scene, but Naval Piranha cannot be skipped and must be fought this time in order to progress to World 5.
Naval Piranha reappears in level 4-S, where it chases Yoshi through an auto-scrolling level. Yoshi can throw a yarn ball at Naval Piranha to make it disappear temporarily, though this does not stop the auto-scrolling and it reappears after a few seconds. It is unknown whether this is the same Naval Piranha as the one fought in the boss level, as it has red vines as opposed to the boss' green vines. If it is able to gobble up the Yoshi while he is moving through the level, he will be instantly defeated.
Naval Piranha also reappears in the Boss Tent. It is fought the same way; however, it attacks faster making it harder to hit it.
Mario Party Advance
A Naval Piranha makes a reappearance in Mario Party Advance. At the Bowser Lab, choosing to see the "scary monster" that he built unveils his Naval Piranha. Afterward, Bowser has the player face it in the minigame Koopa Kurl. After winning, the player gets to keep the minigame. The end of game credits say it now lives and thrives in the desert.
Names in other languages