A water supply device for divers includes a cylinder connected to a mouthpiece and a piston member is movably received in the cylinder so that when the piston member is moved by pushing a push rod connected to the piston member, water in the cylinder flows out from the cylinder and enters into the mouthpiece. The piston member has a central passage defined longitudinally therethrough and has a threaded portion defined in a periphery defining the central passage. The push rod extends through the central passage of the piston member and has a threaded section engaged with the threaded portion of the piston member. The push rod is disengaged from the threaded portion of the piston member and received in the cylinder when the push rod is not in use.