Plasma-induced water treatment is a novel water treatment technique that has shown high efficiency and flexibility. Although the electrical conductivity of impure water varies depending on the degree of pollution, its influence on plasma treatment efficiency is not well understood. In this study, we investigate the fundamentals of a microwave plasma jet submerged in water with electrical conductivities (σw) ranging from 10 to 10,000 µS/cm. The plasma characteristics, namely composition, electron density, and temperature, and their variations as a function of σw, are derived using optical emission spectroscopy. The plasma-bubble dynamics is investigated using space- and time-resolved high-speed imaging. The results show that the plasma fills the bubble volume at relatively low flow rate (typically, < 2 L/min) and high σw (typically, > 1000 µS/cm). The influence of σw on the degradation of methylene blue, a standard water pollutant, is also assessed, and the obtained results indicate that the plasma becomes extremely inefficient at high σw. These findings are of great importance for the community of plasma-induced liquid processing, particularly wastewater treatment.