Actually ! You did not cheat !
The normal way that Waterlines and Boot stripes are done on any model , unless it's molded in , is this . The model is supported in a position that's level , side to side and fore and aft to where the lines should be .
Then here's the rub , You get a good soft lead pencil Or a Sharpie marking pen Medium point . Take this and rig a block you can hold , That is the right heighth when the pencil or Sharpie is on the hull . Now take the block and slide it to the point where the writing tool touches the hull at the bow .
Gently slide it along the hull following the hull's curve . This has worked for hundreds of years and still works now . When done you will have what you need .
I use the Sharpie because under a coat of Acrylic Semi-Gloss it is as black as paint . I picked this up from all the car guys I know , that use Sharpies to do the black stuff around today's modern car glass .
It makes Boot striping easier And you can also ( if you are brave ) use a scribe or engraving tool . Mark the hull with a scribed line instead . Then tape off and spray or brush paint the lines freehand .