Yeah, both waterboxes and windboxes kill knockback. FLUDD can be used for some nasty early gimps because of this too, although it can be a bit tricky.
Also kind of basic, but FLUDDing opponents randomly is much more useful in teams. I've actually won a few friendly team matches by letting a Lucario 2v1 while spamming FLUDD to disrupt spacing and make Lucario broken. By messing up their spacing, I mean keep FLUDDing them as they attack/move around. Either they don't hit anyone, and all you did was waste a FLUDD and reset the position, they accidentally hit/get hit by their partner, they get FIHL which your teammate can punish, or make them get in the way of each other, hindering their general strategy. This is doubly effective if one of your opponents is Snake.
In case you couldn't tell, I LOVE FLUDD in teams. Sadly since my main teammate is a TL, I rarely ever go Mario in teams outside of friendlies, since Kirby is so much better for that team.
It also allows double ganon to do the most epic team combo ever: Utilt>Utilt>repeat x10. Now I know that team is **** and the setup is hard as hell, but come on, that's just awesome.