Redo, partially

Yeah. I tore off the masking setup with great excitement. It was generally ok, with some "patch this" areas around the waterline, but not much more. Then I took a look at it from a bit further away.

Do it yourself and save money

In a few places the thing that was supposed to be the waterline looked as if a drunkard had been freehanding with broad strokes. There was just one thing to do: redo. My in-common-law asked why didn't I use a piece of cardstock with a pencil to draw the waterline quickly and handily, that's what he does with his scratchbuilt ships. I hadn't even think of it, I'm not a boatbuilder...

A cereal box worked just fine

I took a cereal box, cut the openings for a pencil and functioned just fine, as far as I could tell. Then I masked the boat following the newly drawn line. Just like the last time I believed it was ok, but I hoped that my eyes didn't deceive me this time.
Quickly I painted the light areas and took these following pics. Those lonely flappy bits were protecting the tower's white lines from overspray, just in case.

Better

The end results of this attempt were much better. Next I'll get to work on the details, like the propeller, antennae, the periscope and the snorkel that's awaiting getting glued on again. At this rate this boat's going to be finished this year!