Vinyl pinstriping tape, this is what the custom painters use to edge their flame paint jobs. This is not the stuff that is the final color pinstripe tape.
Get it at the auto body and paint supply shop. Mine is by 3M, the size is about 1/8 inch wide. It comes in various widths. It is flexible and good for going around the compound curves, like at the stern.
I strike a level line using a waterline tool then set one end. Then pull it taut as you align the middle and end. Place it where it needs to go around the stern & nudge it into place. Burnish it down well. Backfill the area above (or below) with some blue painters tape.
I use this to mask my boot toppings. I paint a black band around the hull at the waterline. I apply a pinstripe strip all the way around the hull covering the black. Then I mask the hull & superstructure above and spray the underhull red. When that is dry I remove the upper masking, leaving the boot mask in place - then mask the underhull and spray the hull/superstructure color.
When burnished well there is next to no creep of paint under the edge. It removes easily with minimal adhesive residue.