Maybe this will help, GYL
When I bought my DaySailer, the bottom was already painted with anti-fouling paint. It appears that the previous owner did nothing special to prepare the surface for painting, and the paint is sound. On my boat, the waterline is 27-1/2" from the underside of the gunnel/guard strip at the bow, and 14" from the same at the stern. There is a 2" white boot-top above the painted waterline, and the rest of the freeboard was painted blue, which I will be re-painting soon to another color. On hulls that I need to mark the waterline on, I set them up level as I can, fore and aft, and athwartships (a good eye helps here). Next, I make a mark at the bow and the stern where the waterline is supposed to be (as I indicated above). Then I find two stakes, a 4' level, and a ball of carpenter's string. I drive a stake next to the bow, tie the string to it level with the mark on the boat, then do the same at the stern, drawing the string tight as I tie it off. At intervals along the hull, use the level (from the string to the hull) to mark off the waterline. And don't forget about the centerboard and the rudder. I hope this helps.
