How long are you there for? Like the other guys said, there are a couple surf shops within walking distance from Old San Juan and then from there it will probably be like a 20-30 minute walk to find a break.
The cruise ships dock on the south side of Old San Juan and you have to go to the north side to get to the beaches. There's a big fort up there called El Morro at the North western most tip of Old San Juan so anywhere east of there and you'll find something.
Be careful though, theres a couple little slum towns between Old San Juan and the beach so just make sure you steer clear of those and youll be good. PM me if you want any more info, I've been there a few times.