Water injection (WI) is a technology that was widely investigated as an option for solving the issue of knock in boosted downsized GDI engines and has regained attention when hydrogen fueling revealed specific abnormal combustion phenomena. Apart from adverse effects like corrosion and lubricant contamination due to long-term use, an increase in water-fuel ratio can affect cyclic variability and combustion stability. Ion current sensing represents a potential solution for real-time control during WI mode. In this paper, the combustion process is investigated by sweeping the amount of water injected in the intake manifold of a single cylinder GDI engine, optically accessible trough a quartz window fitted on the piston crown. The ion current and in-cylinder pressure-related parameters are correlated with natural emission luminosity and chemiluminescence. The experiments permit to correlate cycle-to-cycle the ion current signals with the chemical markers of flame propagation. Additionally, specific effects induced by the presence of water in the cylinder were linked with ionization reactions. Thus, ion current can be used a reliable sensing method for monitoring the combustion status and water injection process simultaneously.