Made some landscape, added some VK Waterbody Swimming Pools. This generated a WaterZone box around them.
Later on I added a huge VK Waterbody Ocean C and set the spline component at the centre of it to circle around the outside of the landscape with the pools.
Where the ocean spline passes inside the swimming pools waterzone box, that region when in a launched session has no visible ocean surface but player can swim.
Possibly the waterzone of one the two different types of VK waterbodies is interfering with the other.