Now, call me Mr Picky, but I hate it when waterlines are not straight or actually following the, er, waterline when the boat is afloat.
It’s particularly tricky with clinker hulls to get it right, especially when there is a sharp return under the quarter. Many choose simply to follow the clinker lines, which always looks like a cop out.
Now I suppose if I had lofted the hull from the plans, I could have marked out the waterline then, but I came up with a smarter way.
Assuming the upturned hull is levelled correctly (see use of Bekvam stools), then I used a builder’s laser level to project a line onto the hull, and then apply Frog tape along the line. I chose a waterline a couple of centimetres above that on the plan, so the white underside would be visible. This helps to make the boat look less tubby in the water.
You can just see the laser line on the hull
Projecting on to the transom helps get the hull level 
Simply move the laser round to complete the waterline
Once masked off, the real painting can begin. White below the waterline, and grey above.
The finished article. The waterline’s a good’un!