Projected grid, move mesh along xz-axis

I am trying to fix some issues with my projected grid ocean.
One big issue is the waves near the camera creates holes when the edge-vertice closest to the camera moves into the frustrum.
I would like to move the grid back abit, “behind the camera”, along the xz-axis to solve this, something like:
gl_Position.xz -= 5.0 * viewMatrix.xz

But I don’t understand how.
Any advice?

delets faul input, delets faul input, delets faul input

I see that my question is not very well formulated, it’s partly because of lack of understanding of the code I am using and partly beacuse of lack of terms.
By xz-axis I should probly say along the xz-plane.
I only want it to move “backwards” according to cameras rotation on that same plane, and ignore the cameras “pitch” since I want the water plane to stay on Y = 0.
The viewMatrix comes from the OpenGL tools I am using, so I have no idea how it is created.
It may be I need to manipulate the z-vector of the water plane before I perform these?
gl_Position = gl_ProjectionMatrix * viewMatrix * myWaterPlane;

This is almost the solution, but it also changes the y position when pitching the camera up/down:

vec4 preCom = viewMatrix * vec4(, 1.0);
preCom.z += 10.0;
gl_Position = gl_ProjectionMatrix * preCom;

How can I apply only viewMatrix rotation and move the plane along z-axis?