Matosinhos is Porto's main beach. A long stretch of fine sand backed by apartment blocks, an esplanade and any number of small cafes it is pleasant enough. In the summer there's always some sort of activity on the beach and the lifeguard service makes for safe bathing.
The water quality has improved greatly over the years, although it still isn't up to Blue Flag standards. Given there is a large port and oil terminal at the northern end of the beach this isn't a huge surprise.
Matosinhos is also Porto's most consistent surf spot. Unlike the beaches towards Foz there aren't rocks everywhere and the harbour wall at the northern end can provide shelter from the wind. The gently sloping sand here takes some of the power out of the surf making it a suitable spot for all levels of surfer.
One major benefit of coming to the beach at Matosinhos is this area is renown for its great restaurants, particularly seafood.