They're not wearing wetsuits?
Seriously, looking at the whitecaps, the wind is directly over the port rail and not gusting because the seem uniform throughout the picture. Hard to tell from the boom, but looking at the mast, seems like he has 2 reef points in the main. Looking at the wake and tiller, he's making no effort to fall off or round up so I'm assuming they've decided the boat is balanced just where they want to be to ride the rail [toe at this point though could be the handrail if the wind picks up]. As others have said, he could get hull speed with less water in the cockpit if he rounds up a little takes some twist out of the sails