I am not sure if your Mother Earth example fits. That sounds more like it requires both elements to justify the theme rather than the theme existing separately in each element. (It sounds even more like it is a Mono Life theme but not a Mono Water theme.)
+ 
generate a great quantity of creatures at same time. Life on Earth started inside the water, and it's evolution too.
The same holds for your + 
example.
This thread is about or not + . The thematic intersections rather than unions if you will.
Holy water? Baptism? Is theme of purification by water
This thread is about
or
not
+
. This is a critical distinction. That said, you have convinced me about
or
having armies of creatures.
If you had left it at purification I would agree with you on
or
. It was the broader themes of spirituality, sacrality and redemption that I thought required holy to be added to water.
/ 
is superstition, legends about curses of the ocean and creatures. Is the deepness of the ocean
How can legendary water beasts be a Darkness theme? A legendary water beast would never be a Mono Darkness card.
Instead of "deepness of the ocean" perhaps "obscuring vision" with algae (water) or with darkness (darkness)
Superstition and Curses about the ocean would fit in either Water or Darkness
Thank you for your contributions and your explanation of them.
