Any moving mass represents kinetic energy that has the possibility of being tapped to create electrical energy. Air near the surface moves fast, but its density is low (not much mass per m3) so it takes a big turbine to get much energy out. Water is 1,000 times as dense as air so offers 1,000 times more energy per unit area (for equivalent speeds). Three European teams are working to tap moving sea water in two forms: periodic tidal flow and persistent underwater currents. The tidal motions are tapped with a 180-ton hydrofoil on a cantilever that oscillates like a whale's tail and runs a hydraulic pump as the tide comes in. Underwater currents are tapped by use of conventional-looking 3-blade turbines running at much lower speed than wind turbines but generating substantial energy due to the higher density of sea water than air. Neither method is cost effective currently, but large-scale deployment could bring down costs.
Reference
Stone, R., 2003: Norway goes with the flow to light up its nights. Science, 299, 339.