there are a lot of possibilities here, and because you gave incomplete info we can't even guess which one is true or not:
wrong map size for parallax picture - please tell us both your map size in tiles and parallax size in pixels, if they do not fit the background may move with the player without you realising that
plugin is disabled by an incompatibility with another plugin or by wrong plugin order etc. Please show us your entire plugin list in screenshots to check for that.
EDIT: and you forgot to disable the parallax function itself by setting a ! at the beginning of the parallax filename - you can never use the background picture as a map as long as parallaxing is active.
I'm a bit conflicted. One the one hand, it's quite visually cool to make bodies of water that the player can interact with by going into it. However obscuring the player character from a gameplay perspective is pretty bad. Hmm... There's the option to make the area around the player visible, but then that kind of defeats the purpose of submerging them.