El-Jadida is favoured with beaches both to the very north, and south of town centre. The northern stretch is generally considered the most attractive, but both can get really crowded in summer months. The northern beach continues virtually all the way to Azemmour, 16 km away. These beaches are perhaps not very clean, but not dirty either.
There are many, and cleaner, beaches further away. If you want to go for a good swim while you're in El-Jadida, you should really consider the bus ride out to Sidi Bouzid, with its excellent beach that never seem to run out of space for its visitors.