Nasty one.
I had a situation a couple of years ago when I lost my ski in surf near Melkbos. We'd just done a Milnerton to Melkbos in about 30kt and we cut the corner and I got taken out in the kelp at Melkbos, about 300m from shore.
My ski disappeared in the direction of the Koeberg Power Station (recovered it, smashed 2 days later) and I was left with a long swim. A friend tried to give me a lift on the back of his ski, but it was totally uncontrollable, and simply weathercocked into the wind no matter what we tried. I ended up swimming - a long way in cold water.
So it depends - if the wind is either offshore on onshore, you might be able to put the guy, lying flat on the back deck. But if it's 30kt parallel to the shore, as in my case, you're not going to be much help to the swimmer. You can stay with him though and try to get as much of his body out of the water onto your ski to slow down the onset of hypothermia.
I'd try hard to get him back onto his ski first though - as Nige says, you can then raft together, giving the casualty help to stay upright and then you can paddle, one on either side... maybe.
Call for the NSRI early though - they'd prefer to have been called prematurely and then told that the situation was resolved than to be called too late.
Rob
Currently Epic V10 Elite, Epic V10 Double.
Previously: Swordfish S, Evo II, Carbonology Zest, Fenn Swordfish, Epic V10, Fenn Elite, Red7 Surf70 Pro, Epic V10 Sport, Genius Blu, Kayak Centre Zeplin, Fenn Mako6, Custom Kayaks ICON, Brian's Kayaks Molokai, Brian's Kayaks Wedge and several others...