I know that there is a AO being done. But I don’t know how you can access it . it is available as a buffer. It would need to be inverted and have its contrast adjusted but in theory should provide a perfect masking solution for object penetrating the water. the other solution would be to do the math and calculate it yourself to give you more control.
I did find this. But honestly it is way over my head. http://www.gamedev.net/page/resources/_/technical/graphics-programming-and-theory/a-simple-and-practical-approach-to-ssao-r2753
Not sure if that would work though you are prolly going to need a solution that avoids self occlusion and only calculates for other objects.
Also note that this is not practical for the shore line and that the depth based approach is prolly more effective there.