Prior to painting the hull, I had a go at marking the waterline last night.
So that I will know where it is through the first coats of primer, I decided to bite the bullet, measure it from above, below and sideways to make sure it was in the right place and then lightly scored it into the glass fibre.
First I marked out on the hull the exact position of the waterline at the bow and the two rear quarters. Then using these, I set the boat up square on it's stand on my flatest worktop so that the three marks were all exactly the same height above the bench. I put masking tape around the hull/stand join to keep the hull in that position. I used an engineer's combination set with a dental probe attached, which sat squarely on the bench and was able to slide around the clean worktop scribing a neat line into the hull.
I then scored a second line above this to mark out the white boot-top stripe between the two. This setup can reach into the area around the hard chines forward, giving the correct width for the boot-top in that undercut.