I was actually surfing just south of the Manhattan Beach pier & sitting on my board. There was an object that caught my eye. I thought it was a small drone at 1st. It was about 50-80 ft above me but had NO propellers & made no noise. I thought maybe a plane or a bird but it didn't move for about 5 seconds. Then it moved to the side and back. It slightly dropped a couple feet. Then stood still. I was pretty confused on what it was. Then after about 20 seconds I watched it take a fast dive into the water about 15 feet from me. It splashed in & I saw the trail of bubbles sink down with it. It never came back up. I thought maybe someone else would've noticed but I was the only person out surfing. Maybe someone on the beach was controlling it and accidentally crashed it but no one was looking in my direction. There were only 2 couples on the beach as it were. Idk exactly what I saw but I did see it very clearly. It was disk shaped, dark gray, looked smooth as silk not metal. It moved unlike any aircraft that man can create & seemed to be able to navigate its way through the water.