This could go one of two ways, and my partner in this can be male or female, it doesnt matter.
My character could be the owner of a yacht, travelling the oceans for a race perhaps, or just to get away from everything. I could be seen by a mermaid, eager to alter me from human to a mermaid to have some company, or to keep her race going, or by a merman, to mate with and keep his race going. Either one would do. This could happen in the open ocean, or just off an island that my character stopped off after a big storm.
Let me know which one appeals to you and we can play it out.