Casual rooftop restaurant with floor-to-ceiling windows & a terrace with water views.
It's located in Bica area (between Bairro Alto and Cais do Sodre). The entrance is from a little boutique hotel's elevator. It offers tapas and cocktails. The outdoor terrace has an uninterrupted view of the waters. Food is ok and the prices are moderate.
I would opt out of dinner, but do an afternoon drink there.