I definitely agree, if you cannot do this at some other date as a land vacation, then I would pay the extra just for the peace of mind. All the potential hassle of finding the car pick up point, making your own way, possibly having to queue where the ship may have secured priotity entry & so on, and that before you get there. The you will have to cut the day really short to return the car in time to get transferred back to the ship before it sails.
I hate paying over the odds, but sometimes it's the best/safest way.
Last Cruise: Queen Victoria November 2014
Next: P&O Britannia March 2015
Then: Minerva April 2015
and : Royal Princess August 2015
Golden, Grand(2), Caribbean, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Oceana(3), Oriana(11), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(6), QE, Caronia(2), Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey