The first woman to be a commanding officer aboard a ship of the U.S. Navy could one day be Cmdr. Deborah S. Gernes, 39. In a press conference in San Diego, where Gernes is stationed, Navy officials announced that she has become eligible to receive a command on one of its ships, the first female to qualify for such duty. Gernes expects to receive a commission within three years. In the meantime, she will continue serving as one of six female executive officers in the Navy.