How in the world do you strike a straight waterline on a three dimensional shape like this? First, set the dinghy down on an absolutely flat, level surface. A 30-inch-wide workbench worked for us. Make sure the dinghy is level from side to side (note stick spanning the rails on which the spirit level is checked).

Next, make sure the dinghy is level fore and aft. The bench should be perfectly horizontal for these purposes.

Prop the hull up with scraps of wood to hold the hull level in both axes as needed.

Make up a block (or stack of blocks) exactly 5 inches (127mm) tall. Hold a pencil flat atop this block, and slide it around on the table, marking the waterline.

It works even on the curved sides of the hull.