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...between Cirno and the Watermelone, you first have to understand the nature of Gensokyo and Touhou in general. Looking at a strictly canon perspective, there is no confirmed situation where the Watermelone and Cirno ever met, and certainly not through introduction by Sakuya as the OP image would imply. Cirno meeting the Watermelone is merely an inferred situation: a posibility not set in stone through the word of ZUN, yet neither ruled out as something that couldn't occur through some sort of contrived circumstance. This is likewise why certain works depict the Watermelone as an angry god demanding sacrifice, a benevolent figure from which to channel Cirno's hopes and desires, or a simple snack food to be swiftly consumed. Until deemed true or untrue, any of these outcomes could possibly occur in the hypotetical case that Cirno and the Watermelone were to meet. It is something that is under no constraint, much like Momiji's personality and Reimu's family.
Primaries might scold you for stepping outside of canon constraints and attempting to fabricate a situation they deem contrary to character - "sakuya giving the cirno the Watermelone" would imply a certain sense of altruism on the former's part that has yet to show in canon, leading to the argument of character derailment, but many popular doujin works interpret the exact characterizations of characters differently from strict canon and are not always lesser for it. After all, the canon fighting games take artistic liberties in the depictions of individual characters, yet remain supported by ZUN regardless if they depict his characters in the way he originally invisioned them.
Ultimately, the Cirno and the Watermelone's relationship is dependent on the context of the work you are consuming. In one story, they can be bitter foes, a damsel in distress trying to outrun a blood-thirsty monster in vain. In another, happily married. To understand this is to have a basic understanding of Touhou as a doujin scene.
