First time cruisers often do not realize that the entire ship must be cleared before anyone can embark.  They show up at 9 or 9:30 not realizing that many of the passengers are relaxing in a lounge on the ship until 10:30 or 11 and no one is allowed to board until they are off...

That said, you can often drop the luggage early in the morning and return to the port at your designated boarding time...Unless you have priority boarding, the cruise lines are adhering pretty strictly to the boarding schedule nowadays as it is really making the process much smoother.