It has become common for families, even many in the Haredi community, to want to do things together, and to find ways to make it work, rather than splitting activities among boys and girls. That includes activities like going to the beach by going to what has become known as "family beaches", even though there is no such official category of beach. A "family beach" is a beach that has become known to be more family oriented so the women are not generally found in bikinis but slightly more covered, but the swimming is mixed, though families tend to stay together. And a lot of the women on these beaches are frum women from other families to they are either fully dressed or wear tzanua bathing suits. And the men are often also wearing tshirts and the like. Other activities that have become popular are "kayaking" and rafting, with the many frum vacationers during bein hazmanim a large percentage of the revelers at these sites, and maybe some specific ones are known to attract a higher percentage of a frummer crowd, are frum in these weeks and more appropriately dressed.

Rav Yitzchak Yosef pasked there is no such thing as a family beach being ok. It is assur to go there or to any of these sites that are mixed crowds. It is against halacha, according to Rav Yosef's psak.

Rav Yosef added that nowadays the shuls all have yeshivos bein hazmanim where the boys can learn during vacation (and in many places they get paid a bit to learn), and they should go there and not to the places with pritzus.

source: Kikar