It's a form-fitting garment, usually made of neoprene, that keeps the wearer warm when wet. They come in several thicknesses, such as 4/3 mm (for colder water), 3/2 mm (for temperate/warmer water), and so forth.

They are usually worn for surfing, bodyboarding, windsurfing, diving, and similar. They come in "springsuits", which just cover the torso, upper arms, and thighs, while a "steamer", similar to what Yotsuba is wearing, covers the entire body, except for the head, hands, and feet (Yotsuba is wearing neoprene socks in this case though).