How to waterline Queen Mary?

 what I usually do is very simple and seems to work EVERY time.I make sure that my model is LEVEL to the tabletop.(this is important) now I make sure the ship is level to the proposed waterline. There are differences, but, few. Now ,take a block that allows you to place a pencil on top(make sure it,s sharp)At the level you want.Now take this and place on hull.Draw a line from bow to stern or vicy versy.That line will be the waterline.You can draw it very dark or light to your preference.When you,ve done this,spray the hull with clear craft flat.This will keep you from wiping the line off as you work on the project.Now if you want ,use the x-acto blade method for cutting the hull.I normally use a razor saw .This has to be your preference based on your skill.Then, as the other reply intimated,take a piece of glass ,fasten sandpaper to it and WET sand the hull perfectly flat(no list ,fore ,aft or side to side.) Sand to your desired hull depth.VIOLA! your Done with that part of it.    tankerbuilder

Here's a way you can think over as well:

http://www.modelwarships.com/features/how-to/waterlining-350-kn/index.html

I took a 1/700 full hull model and sprayed the hull with primer and dipped it in alchohol to get a starting point for a waterline. I also know a person who used a laser level to project a waterline and slowly scribed a waterline and then did the tape thing.

What Ed said, but how to measure a level line?  Put the hull on a flat surface, a piece of glass is ideal.  Make sure it's sitting level on both planes, ie no 'list'.   Figure how far up the hull you want your line.  Find a block of wood or something that is that height and place an X-acto blade on top.   You could also use a pencil if it fits better.   Slide the blade all around the hull at the waterline while holding the hull steady, scoring a level line all around. 

This gives you a 'level' line, which isn't the same as a 'straight'  line.  The idea is to have a line that a water level would come up to.  

There are special tools to do this, and I probably could have explained it better.

Fred

PS- your kit may already have a scribed line, in which case you can use that as a reference.

Measure & mark off the waterline allowing 1/16 to 1/8  or so extra (below) to sand & fine tune the edge.  Then apply masking tape along the desired cut.   Position the tape above the cut line to protect the surface detail.  Using the back side of a new #11 xacto blade, drag the blade along the tape with just a bit of pressure.   The blade will plow out the plastic along the tape,  not cut into it.   Repeat.   You really don't need to add pressure for successive cuts as each pass cuts a bit deeper.   Just before you break through add some cross bracing to help maintain the shape.     Finish cutting through.  

Glue some wet-n-dry sand paper to a flat surface (piece of plywood or glass) and use that as a flat sanding plane.   Sand the hull to the desired waterline, vary the sanding method to help insure level (switch ends, sand clockwise/counterclockwise.     Check your progress regulaly and sneak up on your waterline.  Its easier to sand some more off than to sand some more on.