You don't necessarily need a car if you just want to see Wismar, since train connections are quite good. Driving is a good idea if you want to see more than just Wismar. You could stop at Bad Doberan for instance, which is between Warnemünde and Wismar, to visit the marvellous minster, and from there Heiligendamm, Germanys first resort (18th century), is also just 5km away.
To take full advantage of the car I would however recommend to drive to the opposite direction to see Fischland-Darß-Zingst and Stralsund. The peninsula Fischland-Darß-Zingst is one of the most attractive regions of the Baltic coast and not accessible by train. The resorts are quite unique and unlike most other on the Baltic coast. The majority of the old buildings are colorful fishermen's or captain's houses, and in Ahrenshoop, the poshest resort on the peninsula, many of them were turned into artist's residences. Fischland-Darß-Zingst also offers great beaches, and the Weststrand close to Ahrenshoop is often called Germanys most beautiful beach.
Stralsund is a preserved Hanseatic city and World Heritage Site like Wismar, in Stralsund however all churches survived WW2 (and the main church is *very* impressive). There are also some good museums and the Gorch Fock, a three-mast barque, in the harbor.
Here you can browse pics of Fischland-Darß-Zingst:
Edited: 2:25 pm, August 25, 2012