I remember trying the new Sheaffer Skrip Black in a cartridge once or twice in the past few years, including an accidental water test and then an intentional one. The results were still legible. Video11's water test, a ten-minute soaking (far more than the few drops of rain or sprinkler water an envelope would endure in a postal system), also left a faded and slightly legible result.
Today I finally bought a bottle, loaded my A T Cross Century II Fine (a wet writer with a firm nib) from it, wrote with it on some medium-good notepad paper (Ampad Evidence Recycled) and ran both sides of the sheet under the tap for a few seconds each side. The sheet also happened to have the new Skrip Purple written on it. The Skrip Purple started to wash away, leaving only a faint trace. A little bit of the Skrip Black went with the water, and the rest expanded and separated a little in the soggy paper, but it remained very dark and legible. The Skrip Black feathered only slightly when wet; it expanded evenly more than it feathered.