Innate Mesmerizing Voice
The mermaid wasn't too sure if Yale's words were trustful, but she knew that there are people who act for such motives and believed that her own brother would have acted in the same way if he found Yale protecting his little sister and needing help.
She thought that those who were kind with foreigners had a second intention or were stupid because she couldn't believe that someone would help others without any reason.
Of course, due to Yale speaking about his little sister, the mermaid ended putting Yale in the group of stupid people that would help others due to reasons like resembling someone else.
Yale didn't know what that small mermaid was thinking of him, but he didn't care as long as he managed to establish a good relationship with those two. After all, his main reasons for diving was obtaining information about the Sea Race, and although he didn't know if those two were of the Sea Race, they might know valuable information.