There are two ferry harbors in Poland: Swinoujscie (about 110 km from Szczecin) and Gdansk. 
From Sweden: Ystad (7-9 hours, 215 zł) by Unity Line; Karlskrona (10 hours, 140-220 zł) by Stena Line; Nynäshamn (18 hours, 230-270 zł), Visby (13.5 hours, 170 zł), Ystad (9.5 hours, 230 zł) by Polferries.
From Denmark: Copenhagen (9-11 hours, 220 zł), Bornholm/Rønne (5 hours, 125 zł) by Polferries. 

Other useful sections of the website

Tourism in Poland

Travel to Poland