Neat problem, and not an easy one to solve.
I have found that external flow problems are tricky - especially when it comes to applying boundary conditions, and especially for multi-fluid problems like this one, which has water and air.
You may want to try creating a "fluid boundary" that encompasses the problem you wish to solve. It can be a simple box, where the bottom is the sand and the top is the sky. Envision a wind tunnel. This way you'll have an internal problem and it may be more obvious where to apply the initial conditions and boundary conditions for the air and water. I don't know exactly how to solve this particular skimboard problem, but this is how I would approach it at least. Hope this is somewhat helpful!