Last minute cruise deals can be very attractive. If a cruise from Charleston doesn't sell out, the cruise line may drop the price on remaining cabins.
Don't expect the selection to be good. These are the leftovers, and the most popular cruises won't be discounted at all beeause the ship is full or nearly full.
Carnival Cruises' last minute deals are listed on their website. You should also check to see what is available from travel agents and the online travel sites.