When travelling Rostock from central station, you can take the S-Bahn (in the direction Warnemünde) and disembark in Lütten Klein. Then you need to take bus No. 45 (in the direction Warnowblick ) and and disembark at Seehafen Fähre (these run every 30 minutes Monday – Friday and also hourly Saturday – Sun).
If you are travelling from central station south, you can take bus No. 49 which runs via Kröpeliner Tor and Dierkower Kreuz directly to the port and disembark at Seehafen Fähre (these run every 60 minutes Monday – Friday and every 120 minutes Saturday – Sunday).
Bus No. 19 also runs to Dierkower Kreuz and is accessible by tram lines 1, 3 and 4 (these run every 60 minutes Monday to Friday).