Born and raised in Hawaii, Watanabe, is the retired founder of a Hawaii-based law firm bearing his name. In the nonprofit world, Watanabe is the chair of Alliance member Consuelo Foundation, which serves exploited women, children and families in the Philippines and Hawaii. He is a longtime trustee and former chair of the Sesame Workshop, a trustee of Punahou School, and the former chair of The Nature Conservancy of Hawaii.

He chairs Hawaiian Electric Industries and serves on the boards of its electric utility, Hawaiian Electric Company, and American Savings Bank. He is on the board of Alexander & Baldwin and its shipping subsidiary, Matson Navigation Co., Ltd., and additionally serves on the boards of private companies in contracting, insurance, publishing, telecommunications, and biomedical technology.