It's a long way (76 miles). I would suggest letting the cruise line arrange your transfer.
It's about a two hour trip, and you would not want to miss the ship. In this case it is
definitely worth the price.
We are also booked on the August 26 Baltic Cruise. However our flight lands at Heathrow. We are going a day early and will go to Dover either by train or by private car the next day. Can't wait to get on the ship, but don't want to get there and have to wait to board. Where are you flying from?