As already posted, alternate arrangments are made for passengers who arrive onboard after the life boat drill.
The cruiselines take the drills very seriously, and rightly so. They are, after all, responsible for the life of every passenger and crew member onboard. In fact, on one of our cruises, the captain came over the PA system on the morning after the drill and stated how disappointed he was that several passengers had unilaterally decided that they did not need not attend the drill the afternoon prior. He then proceeded to call out the cabin number and name of every passenger who missed the drill, and lambasted them by telling them that they put not only their own lives in jeopardy by not showing up, but also the lives of all other passengers and crew members.