I'm 15 year olds boy , 5'6 and weigh 165 pounds.i scuba dive 2 or 3 times a week and I do a 30 to 1

I'm 15 year olds boy , 5'6 and weigh 165 pounds.i scuba dive 2 or 3 times a week and I do a 30 to 120 minute walk once or twice a week and I dont eat much only 1 meal a day of aprox 500 to 600 calories or sometimes a meal of 600 calories and a small meal of 300 and I drink water and cocacola zero and I'm not losing weight.help!!