Though I do not have a waterline on my CR-914, I did have occasion over the years of model building to do them many times. In fact, I did two versions of it: a simple water line, and a complete hull coverage below the water line.
Geoffrey Langdon touched on it, oh! so very closely, in his reply. The difference, which I think makes my approach more workable, is the 'substance' that you use in th tub to mark the line. Rather than using an oil, take a trip to your local hardware store and ask them for a 'refill pack for a chalk-line'. You'll get a small packet of powdered chalk, usually blue or orange.
Sprinkle or lightly blow the chalk onto the water around the yacht. Note: you MUST let the water in the tub settle for about 15 minutes before putting your yacht in - any little ripples will cause the markings to be uneven. Now, with the chalk spreading out over the surface of the water, it will eventually reach the hull, and will cling to it as you remove the yacht from the tub.
Set the yacht on its rack to dry. When dry, proceed as Geoffrey outlined - one strip of tape above the chalk line to mark the top, and two thin strips of tape immediately below it (remove the chalk after placing the first strip of tape, but before the 2nd and 3rd).
Then, remove the 2nd strip of tape, paint and let dry. Remove the 1st and 3rd strips, bevel, lacquer, etc. The only downside to this approach is if you happen to be married, your wife is gonna kill you when she sees the tub!