Related:How is shooting into water handled? But that's for D&D Next, not 5e, and is for shooting into a presumably fairly deep body of water.
A subquestion of: How does animated water work?
When attacking through a wall that is composed of water (for example a mundane waterfall or the effects of the Shape Water spell's first or second bullet points), what sort of penalty applies? Can water provide cover? What type of cover? Does it apply disadvantage? Does it matter how thick the wall is? Does it matter how fast it is flowing (e.g. Shape Water often results in thick, static walls of water)?
I realize cover determinations are up to the DM. I am the DM. How do I decide this?