Not just a stunning spot for a dip, Piscina das Marés (in Leça da Palmeira, on the outskirts of Porto) is also an architectural marvel. The angular concrete frame was an early work by starchitect Álvaro Siza. In fact, Siza grew up ten minutes away in Matosinhos – which might explain the intimate way that the pools and ramps of the Piscina das Marés melt into the rocky Atlantic coast.