We propose and demonstrate a passively Q-switched Er-doped fiber laser based on pure water as the saturable absorber (SA). The SA is made of two optical ferrules matched with a cannula, and the gap between the end-facets is filled with pure water. The nonlinear response of this SA has been characterized, and stable Q-switching operation at 1558.03 nm has been achieved. The maximum output power is 21.1 mW with 65.0 kHz repetition rate. The duration is 1.44 μs, and the pulse energy reaches 324.8 nJ. To the best of our knowledge, this is the first demonstration of the passively Q-switched laser with pure water as the SA. It provides further evidence of the possibility of liquid as an effective SA for pulsed lasers.