i don't know if you can connect a displacment to a use backgroung, probably yes but not by default options. but you can do the invert thats a solution.
you make all in the scene use background and keep your ocean, render that and invert the matte in compositing.
if i am not wrong you will have a matte like you can use to do what you need to. like removing everyting but you ocean.....
It is not elegant but it will surely work.
let us know if you find how connecting it.
jaz