Your mermaid does exactly what many of us do, living in Florida. Wear sunglasses, light, loose clothing, and apply sunblock regularly. She is your creation, so you have full flexibility and you've given her almost exactly the same abilities as humans - just a personal touch of not having experienced her ability to see light and breathe air (which she has physiologically - but has not experienced).
She is personally unfamiliar with sunlight, walking on two legs, and
breathing air; but her species has apparently evolved to tolerate (to
some degree) to Florida's heat and sunlight, according to your post;
She has access to information and resources upon arrival on the
beach? Or at least knows how to survive - it's not far from a lot of
great fishing spots, shelter is the sea, weather is normally fine (at
least under the water), and in Florida she will not be the weirdest
person on the shore - trust me;
She has a task to attend to?
Her goal is to trade whatever she brought for clothing and sunblock; dry off, and find who she's supposed to meet in your story. If land-walking is difficult, she should get a mode of transportation like a wheel-chair now that she is using legs and breathing with her lungs.
In essence, she might already be here...