Banette is meant to be a suicide Trick Room user, and it's not meant to last long. Its movepool is extremely shallow and it has poor defenses; the latter trait makes it ideal as a suicide Trick Room setter. Also, Banette must be paired with strong Trick Room users like Marowak (this suggestion is assuming you keep Banette):
-Shadow Claw (really, there's no need for Shadow Sneak while Trick Room is active)
EVs: 252 Atk
Item: Focus Sash
Ability: Insomina/Cursed Body
Something that can pair well with Luxray or Zangoose is Alomomola, who can restore their health with Wish. It should go over Dewgong. The only thing Dewgong is good for is Rain Dance support. It's just too weak to accomplish much, and its defensive typing sucks; this gives it reasons why it's not used often, even in NU:
EVs: 252 HP/252 Def/4 SDef
Zangoose's Facade hits Dragons hard enough already, so there's no need for Ice Punch. All it needs to use alongside Facade are three of the following: Close Combat for Rocks/Steels, Night Slash for Ghosts, Quick Attack for priority or Swords Dance to crank up its Attack to even more insane levels.
Here's what I mean:
STABed Facade: 70 (Facade's BP) x 1.5 (STAB) x 2 (Facade's power boost) = 210 BP
unSTABed supereffective Ice Punch: 75 x 2 = 150 BP
Marowak can still function without Trick Room, but it needs support from Choice Scarf users to help take down faster threats.
I said Banette was for Marowak. I knew it was going to kill itself, and yeah, I brainfarted on Shadow Sneak. But basically you just told me to do what I was doing anyway with a different attack. The reason I gave it Taunt over will-o-wisp is so it would kill them faster.
I'm looking at the other water Pokemon, and Alomomola does seem to be the most promising, I'll use it.