Mermabel is the shipping of Mabel Pines and Mermando. It was canon in The Deep End.
Mermando was thoroughly entranced with Mabel, and genuinely fell in love with her. He told her, "I have never met anyone like you". He was the one that gave Mabel her first kiss. Mermando seems to care for Mabel as his only human friend, and Mabel also cares for him deeply, going out on a limb to help free him from the pool and let him see his family once more, even if it meant they may never see each other again. Fortunately, Mermando sent her messages in a bottle after returning home, letting her know how happy he is and how much she means to him.
Mabel and Mermando still had feelings for each other many episodes later. In Sock Opera, Mabel mentions she was still in the process of getting over Mermando. In Society of the Blind Eye, Mermando sent a message in a bottle to Mabel saying he had to marry the queen of the manatees to avoid a civil war. Although some could interpret this as a plain break-up message, the act of sending a letter to Mabel about the situation could be Mermando's way of letting Mabel know the possibility of continuing their relationship was slim even though he still loved her.
Mermando was the only summer romance of Mabel's that did not go completely awry, and Mermando is the only 'normal' person that Mabel dated. If not for their species difference and the arranged marriage, this ship could have remained canon and been perfectly healthy, fluffy, and romantic.