Yes, debit and credit cards can be used, I've used one of each.
Parking is gated and secured, however, there have been some occasional problems with breakins in the past. Haven't heard of any since the Sept 11th increases in security, but I would still think it's possible.
You also need to be careful about not leaving food or open soft drinks in the car since insects can find their way to your car while you're onboard.
copied from the FAQ section at
www.portcanaveral.org :
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Parking at Port Canaveral's Cruise Terminals is paved, gated, and under 24-hour surveillance. Parking fee is $8.00 per day for vehicles up to 20 feet in length and $16.00 per day for vehicles over 20 feet. The total amount for the time the vehicle is to be parked must be paid in full upon entering the parking area. Visa, MasterCard, Traveler's Checks issued in U.S. dollars and Cash are accepted in payment of fees. All fees stated above are subject to change without notice by Canaveral Port Authority.
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hope that helps