Quite a few hotels do deals whereby you can leave your car for up to two weeks for not much more than the cost of one-night's pre-cruise accommodation (e.g. Holiday Inn, about £110). If you go by train the day before (to be on the 'safe side') the Ibis, which is across the road from the station, is only about £50 a night and is very acceptable (if you get a room on the correct side, you can also see various ships at the Mayflower Terminal).
Hi Andy
One of the best things about arriving the day before, apart from avoiding the risk of being snowed in, is not only to call at The Star Tavern the night before, And then up before dawn to watch the ships come in!
Even from the Town Pier, Mary looks mightier than ever sailing up Southampton water - although never could she look as majestic as QE2, and if she's ( QM2) docking at Mayflower or the further one towards the swinging ground - it only takes her a few minutes, you can have an early brekkie on a bench - and it's well worth the wait!
We always stay at the Ibis, however security there has been a matter of concern in the past.
It's essential to store anything of value in the office safe.
Cunard also offer parking (it's usually free for the length of the cruise, depending on the 'Offer' at the time of reservation) at the new terminal.
Rosie