The title is a simile for Tita's passion for Pedro. Her love for him is as hot as water needs to be to melt chocolate. Don't think of the powdered mixes we use but a chocolate bar. When Pedro marries her sister, Tita's only way to express her love is through her food, which somehow magically conveys her feelings. The author considers cooking to be an act of love:
And I am convinced that cooking ... is an inversion of the couple's sexual role. This nurturing that our essence carries, and that our love carries and all these emotions, where we are all contained—this is how the woman can, in fact, penetrate the man, this is how it converts, and the man is the passive one, he receives this, and for me it is very intense and very erotic.
Like water for Chocolate is basically saying that it would be substituting one thing for another. like when you make mole to put water in it instead of chocalate. this relates to rosuara being water and tita being chocalate, pedro had rosaura but it did not work out.