Noseclip

A noseclip or nose clip is a device made of wire covered in rubber or of plastic, and is worn by people during a wide range of aquatic activities such as kayaking, freediving, recreational swimming, synchronized swimming and waterdance. It is designed to hold the nostrils closed to prevent water from entering, or air from escaping. Some types of nose clips also feature a long band to keep the clip around the neck while it is not being used.

...
Wikipedia