The optical transmission characteristics of deep ocean waters show a broad maximum between 450 and 500 nm. The most efficient incoherent sources are electric discharges of elements with transitions into this spectral region. Lamps with emission efficiencies of 6% and with output powers of tens of watts have been constructed. A review of the spectral emission properties of discharges of mercury, thallium, zinc, and cadmium as underwater sources is presented. Efficiency of emission and spectral line widths are shown to increase rapidly with metal number density. The efficiency also increases with electrode spacing; however, spectral radiant emittance is independent of electrode spacing. Measurements of time-resolved spectra of pulsed zinc discharges show a dominance of emission in the blue-green.