Oceans cover more than 70% of the Earth’s surface and represent the largest reservoir of thermal energy on the planet. This lecture will present how solar energy enters the ocean system and how the interaction between light and seawater determines the distribution of heat in the ocean’s surface layer. Special emphasis will be placed on the physical processes of light absorption and scattering in the sea, and their role in warming the water column. It will be explained how the optical properties of the ocean depend on water composition (the presence of particles, phytoplankton, and dissolved substances) and how these changes affect the amount of energy absorbed and retained in the system. This demonstrates that ocean energy is not only a matter of temperature, but also of biological and ecological processes. Furthermore, the lecture will examine how differences in temperature and salinity cause changes in density, driving deep ocean circulation and the global redistribution of thermal energy. Special attention will be given to modern observation methods, including satellite optical measurements and autonomous monitoring systems, used to assess energy flows and the state of marine ecosystems. The lecture will highlight the connection between physical laws, the optical properties of the sea, and ecological changes, emphasizing how understanding energy processes in the ocean contributes to monitoring climate change and preserving marine ecosystems.