apply ocean modifier to existing face/plane I have a plane with a specific shape (made by many vertices).
This shape represents the sea.

If I add an Ocean modifier but that generates a new plane.
I thought by adding a boolean modifier on that ocean square, this works and has now the shape of my plane, but the waves are gone.

(I did not apply the modifer of the ocean because I want to animate it over time).

how to do this?

On the image you can see the ocean (via modifier) and my plane which represents the sea.
OK I found out according to this answer: can I use ocean modifier on any custom shape? 
You need to subdivide your plane as much as possible otherwise it will have no effect. and then you set the Geometry to Displace. 
I used the knife tool to first make a rectangle and do some loopcuts and after this I manually took the knife tool to subdivide the parts that were near my "curly" edges.

My mistake was the subdivide (I tested the Displace but thought it didn't work because the waves were gone).

Hopefully useful for others!