It depends on how you get to Seward. Almost everyone who came a day or so early (as we did) got to go on the Alaska Railroad trip from Anchorage to Seward. These people were pre-checked in at Anchorage and we got to the ship around 4pm. People flying in start arriving at the ship around 1 and the last got there at 9pm (they actually held the ship for a late plane with a dozen people on it. If your friends will be on the train expect them after four. If your friends are flying in the same day, figure and hour at the airport and almost 3 for the bus ride to Seward.
Hope this helps.