Riazor and Orzán aren't your typical spanish mediterranean beaches. The sand is artificial, it's ground granite. The water is unbelievably cold, the current is very strong and dangerous, and then there is the pollution and the masses of algae that feed on it. For these reasons few people dare to swim on their waters. These beaches are great however for surfers in autumn and winter when the water is warmer than the air.
If you're here in summer and wanted to go to a beach I recommend you those beaches more to the east, specially "la playa de Miño" some kilometers northeast (not to be mistaken with Miño river)