This chapter synthesizes current knowledge on ultra-lightweight cement composites (ULCC) for marine and offshore structures, emphasizing fly ash cenospheres (FAC) as the main density-reducing material. The relationship between processing, microstructure, and properties is examined, which includes FAC selection, interfacial behavior, and mix design principles that relates density with strength and stiffness. Consolidated findings reveal linear trends between density, compressive strength, and elastic modulus. With tailored fiber reinforcement, ULCC can achieve high specific strength, and in ultra-lightweight ductile forms, it demonstrates notable tensile strain capacity at low densities and moderate compressive strengths. Durability performance is reviewed through permeability, chloride migration, carbonation, shrinkage/creep, and thermal performance. Under high temperature exposure to 900 °C, residual stiffness degrades faster than strength, and small polypropylene (PP) dosages (~0.2–0.5 vol%) mitigate explosive spalling and stabilize post-fire behavior. Upon examining, LC3-based mixes show significant reductions in embodied CO2 (~41.9%) and energy consumption (~26.7%) along with refined pore structure and improved chloride resistance; however, carbonation resistance can decline as shown by mercury intrusion porosimetry (MIP). This chapter concludes with that merging metal and FRP into a true composite component is not merely a suggestion but a proven strategy for advancing engineering design. It enables the creation of lighter, stronger, more durable, and functionally sophisticated products and practical guidance for marine service.