Lagoons... we love them, they are beautiful, for Europeans it sounds exotic, yet Portugal has a few magical ones right here. For kitesurfers they are a playground to practice and show-off.
Esposende has multiple spots where you can ride, you can catch waves on the Ocean side but inside the bay, you can enjoy flat water. It is beautiful, but not easily accessible: parking is difficult even outside of summer, and you will have to walk some distance.
Foz do Arelho is the most beautiful and the finest lagoon for kitesurfing in Portugal. Easily accessible, it also has showers! And you will find several beach bars and restaurants overlooking the bay.
Lagoa de Albufeira is the only lagoon nearby Lisbon, catching good conditions there is difficult as the wind needs to be strong and have the perfect direction. Still, the spot is small, and can be crowded very quickly.
In between Alvor and Lagos is an estuary, so technically not a lagoon, but there is some flat water to kite.
Also in Faro and Cabana between the sand dunes, you will find some nice flat water.