Introducing floating photovoltaic (FPV) at sea, this chapter makes the case for using abundant ocean space to relieve land constraints while leveraging water-cooling to boost PV efficiency. It details mechanical and electrical architecture: panel-on-float modularity, inter-float connectors, float structures, and mooring and anchoring concepts. It also tackles open-ocean realities: wave-induced motions, salt deposition and soiling, corrosion, and maintenance. It reviews emerging designs and pilots, and outlines mitigations to stabilise platforms and safeguard energy yield.