Just been chatting with dad who spent his boyhood hanging around a now long gone boat yard at Keyhaven and was apprenticed to local boatbuilder and character Bill Adams. They tended to build and maintain traditional heavy clinker boats like the Lymington Pram and Oxybird along with local fishing vessels.
The method used for the waterline was much the same in many ways as previous posts.
Get the boat level and then chose the points on stem and stern where the waterline should be. They would then take a batten and tack it around the boat. stand back and look at it, move as needed and so on.
On new boats they would mark a small groove along the waterline so you could always find it for antifouling in the future.
Hardly suitable for the Jolly boat Ed but quite interesting.
Dad did say that some of the more experienced chaps did it by eye but if they got it wrong by either method there was a fair bit of ribbing(pun not intended) on the river as it was there for the whole year
for all to see.