I'm 50/50 on it being a shark. Resolution on the video is poor and there are reasonable suggestions about what else the "shark" could be.
I will say this... he was 18... a high school athlete... and from the video didn't appear to have been in the water long or be overly stressed. I don't think he so much drifted out of view as he's there one second... and then is gone. The video is turning away, but it looks like he was still in frame when he vanished.
Either way, I can't imagine his terror.
I went to Costa Rica, once. A beautiful, clear, and sunny day. Learning how to surf with some friends. We were bobbing along with our boards in about 10 feet of clear water with the sun almost right above. In order to check out some other surfers, I switched the side of the board I was hanging on - which now put the sun slightly behind me (instead of in my face). The effect was that the magnified cylindrical grey shadow projecting onto the ocean floor below me... which I had previously not seen because of my positioning... became suddenly and terrifyingly apparent the first time I looked down. It was either Jaws or the shadow of Jaws... my brain knew it, my body knew it. If was one of, if not the, scariest moments of my life. I still remember the kind of instant chills/adrenaline rush... maybe it was a prey feeling, I don't know.