What is the ruling on a woman swimming in a mixed-gender beach while wearing very modest attire?

A Muslim woman's swimming is Islamically permissible under certain conditions: covering the 'awrah (private parts), not mixing with men, ensuring other women present are also covering their 'awrah, the place being secure from men's view, and the husband's permission. Swimming on a mixed beach is not permissible, even if the attire is modest.