Noon (or maybe a little before) is the earliest I would book my flight for in your situation.

- The itinerary may state that disembarkation starts at 8 a.m., but that is not guaranteed. I was once on a cruise where disembarkation was delayed by over an hour due to an issue with another passenger.
- Customs usually does not take long, but there are no guarantees.
- Traffic can get bad between the port of Venice and VCE. IIRC the drive is less than 10 miles but can easily take half an hour on the morning of a cruise disembarking; taking a boat from the cruise port to the airport is at least that long.

Flying out of FLL after a cruise, I have been willing to book flights scheduled to depart three hours after disembarkation - but that is with early disembarkation, an easy (5 minute) ride to the airport, and going to a flight on an airline with a "flat tire" rule (unlike most European airlines IIRC).