In this chapter we consider two significantly different technologies, neither of which to date have progressed beyond research and development activity into profitable commercial application as energy supplies. However, should such R&D be successful, then the potential is widespread for relatively large-scale installations. The common link is variation in the ocean, one of surface temperature and the other of salinity. Both depend on well-established science, but both have considerable engineering challenges to overcome before becoming established industries. Most of this chapter (§13.2 to §13.6) deals with ocean thermal energy conversion (OTEC) as this has been widely studied; §13.7 outlines the principles of osmotic power.