A moored floating or shore-based platform 11 is provided with a plurality of different renewable energy converter systems. The systems may include wave energy converters 12, wind generators 14 and 15, solar collectors (20, Fig 4), sea current turbines 16 and ocean thermal energy converters. The power generated by the systems may be cabled to nearby power grids or may be used in the hydrolysis of water to form oxygen and hydrogen for fuel cells. The platform may be hexagonal in plan view and several platforms may be joined to form a larger integrated structure (Fig 7). The platforms may provide facilities (29, Fig 5) for fish farming and may incorporate housing 35, warehousing and industrial units. A raft of joined platforms may be used for coastal protection and/or as a harbour for ships 34.