The French port of Saint-Malo has the high­est tides in Europe: the sea here can rise more than 40 feet in six hours. At high tide, the only trace of this huge tidal pool is its div­ing board ris­ing above the waves – but as the tide retreats, it leaves a still expanse of water just beg­ging for a dip. After a few lengths, sit on the sea wall and watch the boats sail by.