Water plane and normal water interaction

Apart from the jello water that should be fixed by #635, using water plane creates some artifacts when interacting with normal water.
When the water plane is set at a different height than the normal water, we get a "double submerging" effect.
Water plane above normal water :
(Visible on the left island)

Water plane below normal water:
(Visible on the left island)

When the water plane is at the same level than the normal water, there is still an artifact where the octree stops
Water plane at the same level:
(See the top right corner of the image)