I tend to mask waterlines with what I call a "mixed media", or variety, of different types of tape. Presently I am in the process of building up Dragon's Mobile Bay CG and Panda's DDG Cole and am using Scotch Plastic Tape, 1/8" wide, for the curved areas and Scotch's Magic Transparent tape for the straight areas. The Scotch Plastic Tape, which is blue, is available through Micro Mark. Once I get both types of tape down, I use Saran Wrap to cover the area I don't want painted. E.G., I painted the bottom of the hull red first, then masked off the area for the black stripe. I attach the Saran Wrap to the blue tape and Magic transparent tape with small strips of transparent tape - about 1/2" long by 1/4" wide overlapping each strip. This prevents overspray on the red hull. Its tedious, but the effort is worth the results. Of course, I take off some of the tack of the Magic transparent tape by placing it on my forearm before applying to the ship. This will prevent peeling up the paint when it is removed. I have found the blue Scotch Plastic tape to be low tack and unless the paint being masked is very fragile, does not need the forearm treatment.
Hope this helps.
Dick McC