I like it. They could have met during one of her times topside and he was in her mother's kingdom for some simple ambassador mission. He would have been instantly attracted to her. Then after seeking her company more became deeply infatuated. Then after he learned that she was half mermaid he suggested the marriage, for diplomatic reasons. It would have taken some convincing to get his parents (king & queen) to approve it, but he won out with the promise of strengthened ties with the other elves and gaining an alliance with the merfolk.