Today's art experiments have identified the following: when diluting for underlayers I prefer Rohrer and Klingner SketchINK over Octopus Fluids Write and Draw. It's easier to control and get a smooth result. The latter also dries a little more... granular, I guess?

They are both good waterproof inks for sketching!! I just definitely prefer the SketchINK if I need a diluted underlayer that won't budge.