The usual problem people have with flying to FLL instead of MIA for a Miami-based cruise is the transportation. A taxi could wipe out most of what you saved in the air fare. The only times we have used FLL for Miami cruises was years back when we let the cruise line do the air and they provided the buses.
Carnival wants to move everyone off the ship as quickly as possible. You will probably arrive earlier than 8am anyway, and I'd expect to be off the ship by no later than 10am or so, with that being the last groups of passengers. Nothing is open other than the breakfast service and cash-only soft drinks at one bar, so it isn't as though you could do anything other than sit.