Praia do Carneiro is the closest of Porto's beaches to the mouth of the River Douro. In fact the old breakwater and lighthouse (Farol de Felgueiras) here mark the southern end of the beach and the entrance to the river. Behind the beach is the impressive Fortaleza de São João da Foz, a 16th century fortress built to defend the mouth of the Douro.
I wouldn't really describe Praia do Carneiro as the kind of beach you would want to throw down your towel and spend the day on. The sand is coarse, the water quality isn't perfect and there are better beaches further along the Foz stretch. However this is an interesting spot and a great starting point for a promenade along Porto's fantastic sea front.
Praia do Carneiro also comes into its own on stormy days when waves crashing over the lighthouse make for a great photo opportunity.