It would be much more accurate to say the flight is scheduled to get into LaGuardia at 12:08 p.m.
When the flight actually arrives and when you actually get your luggage might be a different story. If your itinerary includes a connection, the risk is even higher.
Even if the airline could guarantee that the flight(s) will be on time (which, of course, they can't), I'd still be uncomfortable with a 12:08 p.m. arrival time if the ship sails at 4:00 p.m.
I've had to wait an hour for luggage at LaGuardia and I've sat in bumper-to-bumper traffic trying to get from LaGuardia to Manhattan.
With a regular vacation, such as a WDW vacation, a late arrival just means you've missed a few hours of fun at your destination.
With a cruise, a late arrival can mean that the ship sails without you.