For Alabamians, getting to a port is an easy and inexpensive drive to Mobile, from which several of Carnival’s southbound, roundtrip cruises embark. Onsite parking is plentiful, with a daily fee.      Four-day cruises depart on Thursdays and return on Mondays, visiting Mexico’s resort city Cozumel.      Five-day trips to Mexico departing on Mondays stop in Cozumel and Playa del…