And I'm just going to reply here in addition to Chie (even though Yuuna's already met Merrill)

Either one of my girls would prooobably eventually find their way down to the beach or island, and Yuuna would be happy to humor Squid Girl's shenanigans (and maybe use her to play pranks), while Rei would be silently freaked out by her. Especially if ink was sprayed her way.