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In the Netherlands too, if there is a barrier and it’s marked as such (many are mainly signs to keep out jetskis and powerboats) you’re not supposed to go beyond it and a lifeguard (if it’s a guarded beach, most are not) can tell you to go back if you cross it.

Police in summer regularly patrol known unsafe swimming holes and warn people of the risk of swimming there.
But indeed they have no authority to tell you not to, unless you’re on privately owned land in which case you’re tresspassing and can be removed on those grounds.