Nude on the beach or lake shore – well, small children may do so. What about the others? But, of course, older children, adolescents and adults are allowed to do so as well. If it is not a "managed" beach, where a bathing regulation may forbid this. There are even people, who forbid human liberties and ask for your money to serve you such a interdiction! It would be your own fault, if you pay for and accept such a nuisance.
By the way: The source of that figure is the travel industry, which has projected the booking figures for naturist holidays – of course considering, that one and the same person might book a naturist holiday several times a year.
Also, health aspects are in favour of enjoying the sun and the beach in the nude. Why should you deprive yourself and a part of your body beneficial effects of air and light? And even why the Catholic Church speaks in its current fundamental work of Christian social ethics "Love and responsibility" by Karol Wojtyla, later Pope John Paul II, "of the splendour and the beauty of the naked and uncovered human body", being nude at the beach cannot really be an obstacle at all.