This question as been asked many times & apparently is a lousy journey by train. Cambridge to Harwich is 75 miles, so little over an hour further along the A14.
The car park is right next to the ship - £6 per day, or the train station is also within a stone's throw - not sure how many times you will have to change though.
Hi we have just booked this cruise and are going by coach @£55 each. Works out cheaper than driving @ £100 juice and £49 parking.