In Poland, the cycle pathway Eurovelo 10 runs along the Baltic sea. The section between Swinousje and Gdynia is about 400 km which makes it suitable for 4 - 10 days. Read about daily distances and accomodation options here.
If you would like to extend your trip, you can take a ferry to Sweden and cycle there along the southern coast between Karlkrona and Malmö. There are overnight ferries between Swinousje in Poland and Trelleborg /Ystad in Sweden, as well as between Gdynia and Karlskrona.