Plasma-flux cored arc welding (Plasma-FCAW), as an efficient hybrid arc welding method, features good deep penetration and small spatter. However, due to the complex underwater environment, the welding process is prone to interference. Therefore, in this study, a self-designed excitation device was used to apply a transverse magnetic field to regulate the underwater hybrid Plasma-FCAW, promoting the flexible coupling between the two arcs and effectively improving the stability of the hybrid welding process and the weld bead formation. On this basis, welding process experiments were carried out in water environments with different salinities and temperatures, and the effects of the water environment on the droplet transfer behavior, weld bead formation, and microstructure and properties were studied. The results show that an increase in salinity will shorten the droplet transfer cycle, increase the cooling rate, and reduce the weld bead width. The water temperature has a significant impact on the arc stability and droplet transfer behavior. The droplet transfer cycle shortens at low temperatures, and the droplet transfer cycle is irregular at high temperatures. Both the increase in salinity and the decrease in water temperature will lead to an increase in the content of Side plate ferrite and acicular ferrite in the weld zone, while the increase in water temperature will lead to an increase in the pearlite content in the heat-affected zone, thus affecting the hardness of the weld bead.