Concrete is the most widely used man-made material in the world. Globally, millions of tons of drinkable water are used each year in the production of concrete and its derivatives. Considering the regional and global water shortages in recent years, it is considered that this consumption should be restricted urgently. Seawater is considered one of the alternative mixing waters that could be used as a substitute for fresh water. The interest in this subject has increased in recent years and the number of research projects on the use of seawater as the mixing water for concrete considerably increased. Most of the publications deal with the mechanical performance and microstructure of seawater concrete. Although there is consensus among researchers on some of the obtained results, there are many points where contradictory results have been reported. In this study, the effects of different mixing waters, ion concentrations, and metakaolin substitution on the performance and hydration kinetics of cement-based materials were investigated. The results showed that the dissolution of binder ions improved with higher ion concentrations. In addition, the synergistic effect of metakaolin and seawater maximized the performance and hydration kinetics of the material.