When I look at this card I think about the fairytale The Little Mermaid, and the real version, not the Disney version. Right after the prince marries someone else and the mermaid have given up on the sea, she cannot come back there, and her sister in the water is telling her farewell. She had put all her love and devotion, and made great sacrifices, for someone who wasn't worthy of her. She has turned back into a siren but the water is forbidden to her forever. She feels empty like the cup. The reversed pentacle that she wears says she put the material first before her needs and her spiritual nature.
Or maybe this mermaid managed to kill the prince after all, regained her siren body, and it's his face that we see in the water. But she still feels the same emptiness, loving, giving and making sacrifices for someone unworthy.
There is a snake in the background, the grails animal.
The mars symbol is on the fountain in the back and the scorpio on the tree next to the siren. I mention this because 5 of cups is mars in scorpio.
I want to point that since the deck is heavily inspired by the Thoth, the keyword for this card is DISAPPOINTMENT.