In order to decrease CO2 emissions from the burning of fossil fuels, the development of new renewable energy sources sufficiently large in quantity is essential. To meet this need, we propose large-scale H2 production on the sea surface utilizing cyanobacteria. Although many of the relevant technologies are in the early stage of development, this chapter briefly examines the feasibility of such H2 production, in order to illustrate that under certain conditions large-scale photobiological H2 production can be viable. Assuming that solar energy is converted to H2 at 1.2% efficiency, the future cost of H2 can be estimated to be about 11 (pipelines) and 26.4 (compression and marine transportation) cents kWh−1, respectively.