Practical Performance and Prospect of Underwater Optical Wireless Communication - - Results of Optical Characteristic Measurement at Visible Light Band under Water and Communication Tests with the Prototype Modem in the Sea - -. - Takao Sawa; Naoki Nishimura; Koji Tojo; Shin Ito (IEICE Trans. Fundam. Electron. Commun. Comput. Sci., 2019)