In this paper a series of geochemical investigations are reported with respect to calcium contents in different waters collected from the sea off the coast of Cananéia, the lagoon surrounding Cananéia Island and from Nóbrega river (one of the saline branches from the lagoon, also called "marigot"). An almost direct relation of calcium to chlorinity, 0.02168 in average, similar to that of the outer Atlantic water, 0.02156, was observed in the coastal water. This indicates that the terrigenous supply of calcium might not contribute very much to the calcium content in the coastal water. The inflow of land water to the lagoon of Cananéia does not increase the calcium content of the lagoon water, where a slight depression of the Ca/Cl gradient (namely 0.0206) was observed. This characteristic of the calcium distribution is more remarkable in the water of the "marigot", branching off from the lagoon. An extremely low gradient of calcium to chlorinity, 0.0177 is one of the geochemical characteristics found in tropical estuaries where active bacterial decomposition of deposited organic materials renders the geochemical properties of the water more acid and more reduced. The low content of calcium in land water, 4-6 mg/Ca/kg, has also of course, a close relationship to this peculiar aspect of the calcium distribution in the coastal water, and further in the Atlantic water off the Brazilian coast.