A coastal reservoir (CR) is a freshwater reservoir to store river water in the sea area adjacent to the sea coast by replacing seawater. A CR is formed by constructing a long oceanic dike to impound surplus water that is flowing to the sea from a river basin. Freshwater from the CR is pumped throughout the year into a series of embankment canals to reach upland areas for meeting agricultural, industrial, municipal, etc water needs along with the required base blows and environmental flows in river basins. The embankment canals also create pumped storage hydropower (PSHP) potential to meet the energy storage requirements for harnessing variable power resources like solar, wind, etc which are economical, clean, renewable, and carbon neutral. The oceanic dike also creates a mega-scale ultra-deep sea harbor along with a coastal highway and railroad. CRs are cheaper to construct compared to land-based reservoirs wherever the cost of submerged land and immovable property acquisition and rehabilitation of the affected population is very high. They also serve multipurpose utilities such as deep-sea harbor, road transport, rail transport, land reclamation, PSHP energy storage, rejuvenation of over-exploited rivers, etc without appreciable overall environmental damage. CR projects are feasible in every continent except Antarctica as there are few mighty rivers with huge surplus water discharges to the sea and also the existence of vast desert/semi-desert areas which can be converted into productive agricultural lands or forest lands with water supplied from the CRs. Greening the desert or semi-desert lands into lush green farms or forest lands would enhance the soil carbon storage and also provide food grains and biomass. The available biomass can be used as feedstock to produce carbon-neutral biofuels to replace fossil fuels which are contributing to global warming. Few feasible CR projects are listed in the paper that can harness nearly 9,000 billion cubic meters (bcm) of water annually and contribute to achieving a carbon-neutral world. To explain the concept of the freshwater CRs and the associated embankment canals, a CR project to utilize the surplus waters of the Brahmaputra, Ganga, and Meghna rivers is considered as a case study in some detail in this paper. The project would create a 360 bcm capacity CR to harness nearly 1,200 bcm of water throughout the year regardless of monsoon vagaries for meeting various water needs in all major river basins between the Ganga and Krishna rivers.