That particular ferry service has not had a good safety record. Due to the winds and monsoon conditions, there have been at least two accidents, where the ferries sank and people lost their lives and cars. Mind you, this happened about 12 years ago when I lived there and have no idea if since then, the service has improved.
Another way of getting to Langkawi is to drive to Kuala Kedah, park your car at the ferry terminal and travel across with a hovercraft-type ferry. It goes at high speeds (can make you sick at times!) and takes about one and a half hours. once you are on Langkawi, you can rent a car quite cheaply.
Hope that helps!