A cruise ship is rarely "quiet," even in the middle of the night. I'm not saying its impossible to toss someone overboard without being detected, but it would be tricky. The victim would almost have to be sedated...because if she was fighting and screaming, the scene would be noticeable. If sedated, she would be heavy and would require someone pretty strong to lift and toss her over.
Not sure about this ferry cruise, but all the cruises I have taken are fairly secure. When you exit and enter the ship, you must hand your card over and have it swiped. When its swiped, your photo will appear on the screen for visible identification. I was coming back on board once, with my husband and our friends, and was stopped at entry. I had pulled up my hair...and my photo had my hair down. The officer asked me to take my hair down for better visual verification. He called over another officer to also verify my photo against my live appearance. It was a bit scary....but obviously, they smiled and let me back onboard.