I don't know if this will be any help or not but I've had some success making boat wakes using that 'distance to object below' node. Since you already have it applied to the water, it should be easy to place a wake shaped object just below the water plane behind the boat. If it's close enough to the surface it should activate that surf effect. I've not messed with animation but it seems to me that if you have animation applied to the water plane, it would affect the wake as well. 

Just a thought....

M