Flexible shoe extending between rear and front ends, between lateral and medial sides, and between the sole assembly and an upper end of an upper. The upper includes an outer shell including an inner and outer layers, the latter having a foot insertion opening with rear and front divisions, the latter extending between lateral and medial edges. A device tightens the upper by bringing the lateral edge toward the medial edger. The inner layer extends opposite the outer layer and in the area of the front division of the foot insertion opening, between the lateral and medial edges. The inner layer is liquid-impermeable and flexible. When the lateral and medial edges of the outer layer are brought toward one another, the distance between the lateral and medial edges is shorter than the portion of the inner layer in the area of the front division or the foot insertion opening.