The "manual" doesn't specify how much water a waterweed needs to be sitting in, but it appears that if a small amount of water bubbles up and reaches for the sky, as it seems to do, the waterweed plant will complain about being too wet.
You can see in the screenshot below, the upset waterweed is sitting in only 156,5 grams of salt water:
But the one 2 tiles away, which is sitting in 180.1 grams of salt water is as happy as can be.
Seems like a bug...
Not exactly sure... See images and description above.