For wave power the Wavehub project is very important, as it will test several different technologies and provide some real insight into costs:
http://media.cleantech.com/1800/uk-plugs-into-wave-hub
UK plugs into Wave Hub | Cleantech.com
Several different methods of power generation will be built there, and run their power ashore together.

Another widely available and relatively energy dense resource is ocean and tidal currents, which are attempting to be tapped using turbines:
http://www.treehugger.com/files/2007/12/gulf_streams_wave_energy.php
Gulf Stream's Tidal Energy Could Provide Up to a Third of Florida's Power (TreeHugger)

Further to the discussion of ground source heat pumps, it should be noted that CO2 air heat pumps are now able to operate to very low temperature levels, and are vastly more cost effective than ground source:
http://www.jarn.co.jp/News/2003_Q2/30620_Eco_Cute.htm
"Eco Cute" CO2 Heat Pump Water Heaters

If considered as part of a total heating package in conjunction with electricity sources, multiplying the heat value of the electricity by between a factor of 2.5 for existing buildings and up to 4 for new builds greatly raises the energy efficiency of that portion of the eletricity used for space heating, so polar PV, wind, nuclear and coal and gas would all operate at rather higher EROEI.