The right snorkeling mask fits tightly enough to seal out water, yet it is still comfortable around your eyes and nose. You can check the quality of the seal by holding the mask against your face, then removing your hands while inhaling slowly and gently. If the mask fits, it will stay in place securely. If you find that you need to fasten the straps very tightly to keep out water, your mask doesn’t fit correctly and will leak or be uncomfortable. The straps are meant to keep the mask in place, not to create a seal.