The durability of cover concrete in the submerged zone for floating offshore wind turbines is closely associated to the performance of the cement type used, which also plays a key role in the environmental impact of their construction and operation. A real-world study is conducted by simulating the concrete cover, submerging cementitious materials at a depth of 27 m in Banyuls-sur-Mer, located on the Mediterranean coast of France. The objective was to identify the short-term interactions between cementitious materials, biofilm developing at their surface, and seawater after 30- and 90 days exposure, with emphasis on, (i) the influence of cement type on the microbial composition of biofilm, and (ii) the influence of biofilm and seawater on the microstructural, chemical composition, and mineralogical changes within the cementitious matrix. After 30 days of exposure, scanning electron microscopy coupled to energy dispersive spectroscopy detected the formation of Mg-rich, S-rich, and Cl-rich zones in CEM I and CEM III concrete, while CEM V exhibited the same zonation after 90 days. These findings were corroborated by electron probe microanalysis. After 90 days of exposure, regardless of the cement type, calcium carbonate precipitated at the concrete-biofilm-seawater interface, predominantly in the form of aragonite crystals, as identified by X-ray diffraction analysis. In CEM I concrete, a brucite layer formed immediately beneath this CaCO3 deposit. The bacterial and eukaryotic diversity was identified using 16S rRNA and 18S rRNA sequencing, revealing diverse and dynamic communities over time. The macrofouling species, marine polychaetes (or serpulids), have been identified and considered to be biomineral in origin.