I feel Golduck might function better as a doubles support mon than a bootleg Swift Swim sweeper. Cloud Nine is good for weather cancelling, and utility moves like Simple Beam, Soak, Icy Wind, Clear Smog, Disable, Encore, Skill Swap, Helping Hand, Screech, Taunt, Psych Up, Light Screen, and Yawn makes it fairly good at disrupting or enabling strategies (*note, Soak sadly fails on a Tera'd mon. Seems GF didn't want that to be broken)
Of course the bigger issue is its stats (highest is 95 for Sp. Atk sadly), and locking yourself to 4 status moves (incl. Icy Wind) means it'll literally be a sitting duck late game. I generally do this:
Golduck
;
Item: Leftovers
Moves;
-Clear Smog
-Power Gem/Hydro Pump
-Soak
-Icy Wind
Tera: Ice/Rock/Water
Stats:
View attachment 587956
Icy Wind is just generally nice speed control, Soak can assist the ally defensively, or ruin the enemy defensively as a psuedo Tera, Power Gem or Hydro miss depending if I want coverage or Stab, and Clear Smog as a less disruptive Haze to shut down setup sweepers or accidental enemy buffs (like Competitive Empoleon, Defiant users, etc)
Tera is either super Water stab, Ice for buffing Icy Wind, or Rock to buff Power Gem
Tier Wise though, especially as of now, F (down to Doubles NU or maybe UU). If people use it more, maybe it can be high F, low E