Another theme could be beauty and darkness. The black water lilies themselves are a symbol of this. They are beautiful yet have an air of darkness around them, which might reflect on the characters and their secrets in the novel.
The main theme could be mystery. It might revolve around the secrets hidden within the setting where the black water lilies are present.
There could be a character who is an artist. Since water lilies are often a subject of art, an artist character might be interested in painting the black water lilies. This artist could be one of the main characters, and through their eyes, we could see the beauty and mystery of the lilies and the surrounding area. There might also be a local villager who has a deep knowledge of the lore and history related to the water lilies, and their interactions with other characters could drive the story forward.
There is often a central female character. She might be the one who has a deep connection with the black water lilies and is at the heart of many of the novel's events.
I'm not sure specifically as I haven't read it. But it might be about the water lilies in India, perhaps their growth, their significance in the Indian ecosystem or culture.
Perhaps the theme of self - discovery is present. The characters in Norwegian novels are often on a journey, either physically or metaphorically. In 'Black Water', the main character might be going through a process of understanding themselves better within the context of their Norwegian surroundings.
One more theme could be transformation. The black water could be a place or element that causes a character to change in the story. For example, a character might enter the black water as one kind of person and come out changed, either physically or mentally. This could be due to the power or nature of the black water in the story.
I don't know as I haven't read it. Maybe it has a gardener who takes care of the water lilies.
Relationships could also be a main theme. In a 'the water novel', characters might interact in unique ways because of the water setting. For example, they could be shipmates on a long voyage, and they need to cooperate to sail the ship. Or they could be rivals fighting over access to water resources. So relationships, whether friendly or hostile, can be an important part of the novel.
One main theme is love. The relationship between the characters is a central aspect, showing different forms of love in a challenging circus environment.
One main theme is scarcity. In a 'water wars novel', water is often in short supply, which drives the conflict. For example, in 'The Water Knife', the lack of water in the drought - stricken landscape makes people fight over every drop.