The Invisible Boatmobile is a common vehicle featured in the Mermaid Man and Barnacle Boy episodes.
It is Mermaid Man and Barnacle Boy's vehicle, located in the Mermalair. SpongeBob claims to know lots about the boat, like knowing where the windshield wipers are. However, when he pressed the aforementioned button, it turned out to be the Origami Button. Barnacle Boy thinks that making it invisible was a stupid idea. The boat mobile was once almost completely painted black by SpongeBob and Patrick, thus defeating the purpose of it being invisible.
It is invisible, but in its real look it is a pink Cadillac. However, when SpongeBob crashed into the streetlight with it, and when it was painted by SpongeBob and Patrick, it looked like a normal boatmobile.