Sorry but there is something that could throw the water level calculations out as in out of kilter and it depends on the size or even existence of ballast tanks on each vessel, and it’s the number of passengers. Just a few difference no problem but say 8, 10, 15 etc and then the weight comes into play. I suspect not a challenge on newer vessels but older ones maybe. I’m no navy architect although I am proud to say they did train me at the start of my working life.