I would agree with nonopr01, the rubber strip along the top of the aerobar would be the contact point with a surfboard I expect. Thule does have quite a selection of brackets specifically for surfboards. The most common simply looks like the pads such as the one here. In terms of load capacity it looks like 165 lbs per pair and Thule claims they are the strongest bars with a vertical load test of more than 800 lbs. Not something I would want to test myself.
I use those bar pads on my Thule square bars and strap my kayak straight to the bars. Just got back from a trip to Florida (I live in Indiana) and had zero issues.