I believe that mitochondria and chloroplast both were originally independent single celled organisms.

So kind of funny that, chloroplast is being "stolen" again by this sea slug.

Somewhat related https://what-if.xkcd.com/17/

The surface is too small and you only get like 4% of the energy you need (Assuming you like being naked under full sunlight all day long. The article is for cows, but I guess the number is similar for humans.) 4% of 2400 kcal is almost 100 kcal, that is the content of a small diet treat or 2 apples.

These sea slugs can survive because they move very slowly. For a human, I think it's not enough energy to survive even if all the activity is to watch TV inside a hot swimming pool.