Hi Hed.
Been surfing all my life. Used to spend winters on the North Shore, which is where I learned to freedive(Sharks Cove on the flat days, epic!).
I'd agree with all thats been said sofar. Target CO2 tolerance with apnea walking. (I wouldnt recomend running with rocks as the Hawaiins do, sounds like a recipe for disaster without a spotter.) O2 tolerance/empty lung work not such an issue as there is usually enough O2 to suvive even a two wave hold-down. As long as you keep your head, and your finger off the panic button.
Freediving has changed my whole approach to surfing big waves. I surf(the paddling part atleast) like I dive ie. slowly. I try to never let myself be caught out of breath. Even if I'm about to be caught inside by a 20' set I paddle slowly and focus on my breathing(slow and deep usually, but in this situation a bit of purging!). I know I can hold my breath long enough if I'm relaxed, so I rather take the devil I know(having the 20'er break infront of me) than the devil I dont(scratching you arse off only to have the lip land on your head).
And when I'm being worked I find I'm a lot more mindfull now of which muscles arent as relaxed as thay could be, definitly something I picked up from freediving.
Also, in bigger surf, I sit on my board during lulls and breathe a bit deeper and from the abdomin(as opposed to lying on my board)
Another freediving trick i have used to good effect is packing. Sometimes when you straighten out on a very big wave and you have a wall of whitewater behind you its best to wait a few second before jumping off your board as I'm shure you know. I used to hold my breath during these few seconds because Iwas afraid that if I continued breathing I might suddenly go down at the end of an exhale with not much air in my lungs. Now I pack during these few seconds. It's safe cause even if you inhale water into your mouth it wont end up in your lungs. Also, the extra bouyancy the air provides maked the swim up a bit easier if youre not wearing a suit.
One thing I've often thought of doing in big surf but havent yet tried is doing a few static warmups on the beach or in the channel. This would give you nearly double your bottom time(or comfort in this case). The warm up effect(increased buffers) should at least last the duration of a heat.
One more thing. In Cape Town where I'm from, I find that if I lay face down in the water for a few seconds at a time(during long lulls), or even splash water on my face I feel a slowing of the heart. Water here is cold though 12-14 celcius.
A session of pack-stretching before paddling out might also help to a small degree.
Even if you dont do any of these things and just dive your confidence should grow. chow
Bevan