But -0.0 isn't.
I don't think that boat is in flowing water in that picture.
Uh... Proof? Explanation? They're supposed to run quickly to get away from zombies if that's what you mean.
This is due to the block being moved, and the player landing on the edge of the block. If the player lands on the edge of a slime block, they take damage, becuase they didn't land on the squishy, cushiony section. This is an intended feature, however, when the block moves the hit box of the edge is thrown off a bit.
How is it invalid?