What is the name of the squirrel who lives underwater in a diving suit?

The squirrel who lives underwater in a diving suit is Sandy Cheeks.

Sandy is an anthropomorphic flying squirrel from Texas who lives in Bikini Bottom. Because a land mammal cannot naturally breathe underwater, she wears an atmospheric diving suit and helmet when outside her home. Her home is the Treedome, a glass-like habitat filled with air, plants, and a large tree.

Sandy first appears in “Tea at the Treedome,” where SpongeBob meets her after she rescues him from a giant clam. She quickly becomes one of his close friends and a regular member of the series’ ensemble. Her character combines science-fiction imagery with distinctly Texan traits: she speaks with a Southern drawl, enjoys rodeo culture, and is skilled at karate and invention.

A common mix-up is calling Sandy simply a squirrel or assuming her helmet is decorative. She is specifically portrayed as a flying squirrel, and the suit is essential to her life outside the Treedome. Visitors to her home need water helmets so they can function in its air-filled environment.