1. Fill the bathtub up to the top (stuffing towels) in the overflow if necessary to get the appropriate depth.
2. Put a few drops of vegetable oil in the water.
3. When you rake the boat out carefully, the oil sheen should still be visible for a while, marking where the line should go.
4. Use very thin 1/32 inch architectural black line tape to follow above the oil mark (masking tape only if you really cannot find the better tape). This is only temporary, as you would want neither the weight nor the turbulence of a tape waterline bulging out.
5. Once the tape is in place, go all the way around the boat placing two or more tapes packed right next to each other below the first tape.
6. Carefully peel off the middle tape.
7. Paint the hull in the exposed area masked off by the two remaining tapes. This assures a uniform thickness to the painted line.
8. Peel off the two remaining tapes and you are done.