Buses travel between Porvoo bus station and Helsinki's Kamppi every 30 minutes or so (€9.80 to €16, one hour). There are also frequent buses to/from towns further east.
The noble old steamship JL Runeberg cruises from Helsinki's kauppatori to Porvoo's passenger harbour in summer (one way/return adult €29/39, child €14/19) and makes an excellent day trip, with various lunch options available. The trip takes 3½ hours each way, so you may prefer to return by bus. The boat leaves Helsinki's kauppatori at 10am and Porvoo at 4pm.