If the ship has been hijacked there will be no people on deck they will all be herded to the lower decks where a group of men can keep an eye on them, and keep them out of view of passing ships if any do pass.
I do not believe the ship was hijacked from sea, it takes a lot of manpower and the element of surprise to take a ship that size without a distress signal being sent. In fact the distress signal was not sent until 4 days after it disappeared, then it was quickly withdrawn. I have been on Cruise ships in the past and I know for a fact that every ship has 4 locations where signals can be sent, 2 if the Bridge was taken. There is no explanation for why the ship did not send or attempt to send a distress signal if it was hijacked.
An inside job appears most likely. Possibly by some of the crew, not the tourist, the crew would know exactly what rooms to take and put the ship in complete darkness in just seconds by placing themselves within striking distance at the appropriate time. Being crew members, no one really gave them a second look until it was too late. Once said ship was taken, a rogue crew could keep the passengers calm and easy simply by acting as Captain of the ship, therefore not requiring a large body of men to watch over the passengers. The loyal employees would be sent to the lower decks, possibly the cargo bay where a smaller group can watch them.
If the above scenario is accurate, then a strike at the Bridge is the most likely course, take out the leadership and the rest will fall like stones. The ship will be secured the loyal crew back in their positions and save for a few men with guns, the passengers would not be any the wiser.
Of course, much like war, things do not always appear what they are.
By the way we still need leaders.