is it permissible for a woman who is dressed tznius to go to a mixed beach? What in general is the source for the prohibition to go to mixed beaches ( for males at least) ?
While the activity and spiritual atmosphere are questionable, there is no prohibition for a women to go to a mixed beach. For a man there is the problem of being in a place of immodesty, which will lead to viewing women immodestly dressed, which is a prohibition in it’s right as well as causing improper thoughts and impurity. This is a violation of the pasuk we say in krias shma “lo sasuru acharay levavichem veacharay eineychem”.
see Rabeinu Yonah in SHaarei Tshuva 3:62