OK, the problem you described with the "bleed" of paint is caused by the paint creeping under the masking tape. If you apply the tape then paint along the edge (black and red in your case) then the "bleed will be the same colour as the hull. When you paint your white line you will not experience any bleed and the white line on removal of the tape will be crisp and sharp.
I don't now whether you guys get my occasional song references or not, but during the last week or so I was down to the waterline
It took some effort, not least 5 (count 'em, five) coats of white until I was completely satisfied with the covering. Thanks to
steam up's advice, which was well worth following, I ended up with only minimal bleeding of the white color, namely around some of the rivets, which was amended in no time at all
I think the white waterline makes all the difference in the world as far as the paint job is concerned. Well worth the effort.
The line is a tad slanted in some places, bit I doubt anyone will notice once she's on the pond.