These days cruise ships are disembarking far later due to everyone having to go through customs and immigration.
On Carnival Breeze they didn't even start leaving until almost 8:45 and the last people off were at 10:30.
As far as arriving early - I don't see why it matters. You still have to wait in the terminal to board the ship (especially with these later disembarkations). I suggest you just chill out for an extra hour on shore.