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The breakdown of water in underwater discharge acoustic source (UDAS) is usually in the mode of electrothermal breakdown. The prominent characteristic of the electrothermal breakdown is the existence of prebreakdown-heating phase. In our experiment, the influences of salt concentration and water temperature on breakdown are tested. Three phenomena are observed: 1) The breakdown time obeys a “double peak” stochastic distribution; 2) the higher salt concentration in water leads to higher probability of nonbreakdown; and 3) the higher temperature of salty water also results in higher probability of nonbreakdown. In this paper, the mechanism of electrothermal breakdown is theoretically analyzed, and two conditions to realize electrothermal breakdown are concluded: 1) enough energy deposited to the gap to ensure “border boiling” and 2) enough remaining voltage on the gap at the moment of “border boiling.” Focused on the water temperature and gap voltage, a semiempirical model for prebreakdown-heating process is proposed. When two heating effects of both ionic current and radiation from the initial arc are taken into account, this semiempirical model provides preliminary explanations to the three phenomena observed in our experiment. Based on this model, a simulation code (CPH ) is developed which can numerically simulate the evolution of the gap voltage in prebreakdown-heating phase. The simulation reproduces the experimental phenomena under the same initial conditions.