Marking the waterline
I happen to have a tool that I purchased from Micromark some time ago that they market as a waterline marker. It’s apparently a repurposed toolmaker’s surface guage, but it certainly works for marking the waterline. This marker uses a metal scribe to mark the waterline, which works pretty nicely on the soft ABS plastic hull.
Most waterline markers marketed today, like Amati’s or Model Expo’s, are fitted with a pencil. Of course, if you don’t have one of these, a pencil mounted atop a block of appropriate height will do.
In any case, the model needs to be sitting so that the waterline is parallel to the work surface. I just used the included cradle to hold the model, though it’s use resulted in a waterline that’s not quite the same as shown on the drawings, but it seemed close enough.