May one lie or walk on the beach on shabbat? Or is making crevices in the sand Melacha when doing these things?

Beach sand is Muktze therefore one should not play or carry it at all. (Obviously, one should also not dig in any way in it). Lying on the beach can be done only in private. If there are other people around it is forbidden to lie on the beach out respect for Shabbat. Rabbi Yosef Weitzen