I had tried something like that a while ago. While I couldn't found any way to have 'kickables actually float like the avatar, you can however create a collider plane slightly bellow the water plane and set it so that it collide with kickables only and not the avatar. (I haven't found yet how to do that with aclScript). That way your kickables will roll on that invisible plane and give the impression to float.. (make sure that the collider mesh is slightly bellow the water plane so that the objects look like they are a little sinked in the water, and not floating right on top of it)
It is not a perfect solution however because the objects behave on this collider the same that they behave on simple ground, so they move much faster than they should move in water and they don't 'bob' up and down. Also all objects will float at the same level regardless their mass or size. But if you use it only for objects that are roughly of the same size (say a barrel and a large beach ball) it works pretty well.