The reason why Hinamori loved eating watermelons is because in Japanese, watermelon is called suika.
The Suika pairing has two people in it: Suigetsu and Karin.
Their relationship consisted bickering over minor issues, which to Hinamori, was kinda cute.
Even though Suigetsu and Karin constantly complained about another, they held onto that small bond.
Because when they are more intimate when the time to part comes, it would be hard to let go.
So they stayed in that sort of relationship—sweet and subtle animosity.
And also, Hinamori thought it better that way as she bit into a watermelon slice.