Marc - you definitely can't make 0830 flight the day you debark. The airport is 1 hour away from the city and the port and you would have to get to the airport before the ship docks. If you stay over, use the Flybus from the hotel to Keflavik, it's efficient and 1/4 the cost of a taxi.

Marcie - With a lot of people from the ship flying Icelandair Saga (business) class it can be a long line for check in so schedule an early bus. The Saga Lounge at the airport is really great so don't fret a long wait before boarding.
Jim