Another scenic viewpoint in Polignano is the one near Grotta dell'Arcivescovado. While the views are similar to those of Belvedere Terrazza Santo Stefano, here you can see Polignano's sea caves even better and more up close, so I think it's worth checking it out too.
If you're eager to explore these sea caves beyond the viewpoints, you can also book a boat tour (with an aperitivo) along Polignano's coastline - linked below.