Hi Dabblingduck.
I can't remember exactly what the hull looked like originally but, if you look at my first pics you will see no lines on the hull so I guess you need to cut at the lowest line, but my pics should give you an idea of where to cut.
You will need to be fairly accurate, as you will have to glue on the strips either side of the top of the hull to make the rail but any sticky-up bits can be sanded into shape.
I used a Stanley knife to score the cut line then just bent the plastic to break it. I guess that it should have been scored then cut but I found that the ABS split along the scored line when I bent it so I took the easy way out.
Good luck with the model. If you need any help just shout. I try to mention tips and mistakes as I go through the build so, hopefully, you will find it useful.