It's usually a broken board or reef rock to the head, leash caught on a reef, or some other incapacitating event that knocks them out and not a pure drowning type situation. Once you get to the level of being able to surf waves that large you know how to be calm during a hold down and hold your breath for minutes and tread water for hours.

I used to surf bigger waves (about 15 feet was my biggest) and they are no joke even at that size. You need to be prepared for 45 second plus hold downs if you wipe out and white water "sinks" where you literally can't swim and just go under because the density of the aerated water makes it impossible to float. "Tomb stoning" is fun too... It is when the board gets in front of you nose up in the white wash and the force of the water drives the board (and you with your leg attached to the leash) straight down and under water as you are dragged forward by the board and wave. Terrifying. A few of those and you lose interest in riding the big ones (at least I did).