A flexible fastening device includes a male fastening unit including a hook member securely looped around a mask strap, and a stud, and a female fastening unit including a clip put on a breathing tube of a snorkel, and two opposite latched members projected from the clip. The clip includes a bent bridge interconnected the latched members. The latched members include an aperture formed between its open ends. A space is defined by the latched members and the bridge and is communication with the external through the aperture. In a fastened position of the male and the female fastening units the stud is lockingly received in the space. Compressing opposite portions of the clip toward each other will deform the bridge, splay the latched members, and enlarge the aperture so as to permit a subsequent pulling of the stud out of the space through the aperture.