If you try an impact driver and it doesn't work, you might just look at removing the entire fitting and installing a replacement, if you have reasonable access to the underside.
If you can't get it open, replace the whole thing.
We cross threaded ours a couple of years ago.
Had a hell of a time getting it open.
We replaced the entire fitting.
Courtney is My Hero
If a man is to be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most - E.B. White