I'm totally late to the party.
First thing first, check to see that you have stitch vertices enabled. I know you've been mapping for years; do it anyway. Next run the "check for errors" dialog box, specifically for unclosed sectors. Unclosed sectors are hell on wheels for making your map look bad and always come up when you don't expect them.
Now, do me a favor and zoom in reeeeaaaaallllly reeeaaallllly close and examine that there linedef. Are you sure there's not a really small sector that's between the water sector and the wall? Check anyway.
If you're seriously out of ideas, I'd be happy to try my hand at fixing it. No guarantees, but I'll give it everything I've got. Which might not be much, incidentally.
I'm not sure if you want to follow my example, but I learned about why unclosed sectors happen by looking through the epically old threads in Editing. Listening to WadAuthor users talk about life before stitch vertices is very informative. Remember, though, don't bump.