The instantaneous velocity of flow in a water bath agitated by a plunging water jet was measured with particle image velocimetry (PIV). The water in the bath was initially at rest and then driven by the plunging water jet. The period from the start of plunging to the moment at which the flow in the bath becomes a steady state is defined as flow establishment time. The turbulence components were used to determine the flow establishment time in place of the commonly used mean velocity components. An empirical equation was proposed for the flow establishment time.