Hawaii is using a resource it has in plenty - water - as the main ingredient in a novel technique of producing electrical energy.

Fifty thousand watts of electricity is being produced almost directly from sun-heated ocean water on a barge off Ke 'ahole Point on the island of Hawaii, Gov. George Ariyoshi has announced.

State officials say it is the first time electrical energy has been produced in this manner outside the laboratory. The self-sustaining process is called OTEC, for ocean thermal energy conversion.

Ariyoshi sent a message to President Carter calling the development a "historic breakthrough" in the search for alternative energy sources. The experimental project is expected to cost in escess of $3 million.

The breakthrough came Thursday when nine 500-watt floodlights and a television set went on, powered solely by heat generated from ocean water, said state planning director Hideto Kno. Kno said 40,000 watts are needed to power OTEC's machinery, leaving 10,000 watts for other uses.

The success of the project proves that ocean thermal conversion is "a workable, renewable, essentially inexhaustible energy resource which can be brought on-line soon," Ariyoshi said.

Kono said the second phase of the experiment, if federal funds come through, is the construction of a testing facility on land.