move some code from Start to OnEnable

Could you please elaborate on this - did you move all the code out of OceanPlanarReflection.Start and into OceanPlanarReflection.OnEnable? I cant get my head around why that would help, but i'm sure theres a good reason!

Would a solution be to remove this code:

            if (OceanRenderer.Instance == null)
            {
                enabled = false;
                return;
            }

So that the OceanPlanarReflections does not shut itself down when theres no ocean?

I cannot say if you solution would work. The problem is, that Start() is only called once. OnEnable() is called whenever it is enabled. So it´s the safe way.

I've put all code in OnEnable() and deleted Start().

       private void OnEnable()
        {
            RenderPipelineManager.beginCameraRendering += BeginCameraRendering;


            if (OceanRenderer.Instance == null)
            {
                enabled = false;
                return;
            }

            _camViewpoint = GetComponent();
            if (!_camViewpoint)
            {
                Debug.LogWarning("Disabling planar reflections as no camera found on gameobject to generate reflection from.", this);
                enabled = false;
                return;
            }
            _camViewpointSkybox = _camViewpoint?.GetComponent();

            // This is anyway called in OnPreRender, but was required here as there was a black reflection
            // for a frame without this earlier setup call.
            CreateWaterObjects(_camViewpoint);

#if UNITY_EDITOR
            if (!OceanRenderer.Instance.OceanMaterial.IsKeywordEnabled("_PLANARREFLECTIONS_ON"))
            {
                Debug.LogWarning("Planar reflections are not enabled on the current ocean material and will not be visible.", this);
            }
#endif
        }

Ok. I'm guessing this still wouldn't work if it's enabled before the ocean, which seems like something we should support, I think