Plasma technology is a highly promising and advantageous technology for liquid processing. In air in-contact with water, plasma produces highly reactive species (ions, electrons, radicals, photons, etc) that diffuse into the water volume and initiate physical and chemical phenomena of interest, e.g. organic and inorganic pollutant degradation. In this study, we investigate the influence of basic parameters, such as voltage polarity, voltage amplitude, plasma lifetime, and air-gap distance, on the properties of a discharge in air in-contact with deionized water. Specifically, we analyze the electrical characteristics, the plasma behaviour at the water surface, water acidity and conductivity, and the decoloration rate of a standard organic dye (methylene blue). The concentration of the main reactive oxygen and nitrogen species produced in water is also reported. Compared to positive polarity conditions, the negative polarity voltages enhance the decolaration rate of methylene blue. For instance, under negative polarity voltages and while applying 4 and 6 kV, the decoloration rate is relatively low (<30%) and reaches 100% after 25 min of processing at −10 kV. The decoloration rate of MB is also strongly influenced by air-gap distance. Under positive polarity conditions, the decoloration rate decreases from ∼80 to ∼0% as the air-gap distance increases from 0.5 to 7.5 mm, whereas, under negative polarity conditions, the decoloration rate is ∼100%, irrespective of the air-gap distance (0.5 and 4.5 mm).