Water Body Custom Actors generate this warning during Map Check:
WaterBodyCustom_UAID_507C6F36B9215C5B02_1871778061 Static mesh actor has NULL StaticMesh property.
To reproduce, simply add a Water Body Custom actor to a level in a project with the Water Plugin enabled.
Stepping through the code for this plugin, it appears that this issue comes from the base Water Body Actor adding a Water Info Mesh component, but the Water Body Custom Actor never generates a water info mesh, so the static mesh property is never set.