Actually there was an experimental power facility here on this island using the thermal differential in deep vs. surface ocean water. Known as Ocean Thermal Energy Conversion=OTEC.
http://www.hawaii.gov/dbedt/ert/otec_hi.html
Also, we have a functional geothermal plant that supplies a significant potion of electric use here.
http://www.punageothermalventure.com/
Other good sources of juice are tidal and wave energy as well as wind, solar and hamster wheels.
