Stats
Mediocre. They all hover around the 80s, aside from SpAtk, which is 95. Not bad, but not particularly good either.
Abilities
Damp: NO.
Cloud Nine: Probably its best ability. Weather teams get screwed over bad.
Swift Swim:- Meh. Speed boosting's good, but screwing their SS team's better. There are many better SS users than Golduck. As in, pretty much everything aside from Magikarp. And that's debatable.
Movepool
Oh dear... Physically, Cross Chop, Aqua Tail, Aqua Jet, Zen Headbutt, Waterfall, Brick Break, Shadow Claw, Iron Tail, Ice Punch, and maybe Mega Kick?. Specially, there's Hydro Pump, Psychic, Psyshock, Surf, Scald, Brine, Signal Beam, Mud Bomb, Focus Blast, Clear Smog, Ice Beam, Blizzard and Future Sight. Setup-wise, there's Amnesia, Psych Up, Screech, Calm Mind, Wonder Room, Confuse Ray, Rain Dance, Refresh, Simple Beam, Mimic, Worry Seed and Endure. Wow, how many moves?Shame it doesn't get the stats to run this properly. Wonder Room deserves special mention, as the best way to get around walls, without having to switch. Swapping defenses is a huge boost.
Set
YOU ARE GOLD!
Golduck@Choice Specs/Scarf/Leftovers?
Cloud Nine/SS
Modest
-Wonder Room/Brine
-Psychic/Psyshock
-Surf/Scald/Hydro Pump
-Clear Smog/Focus Blast
Simpleish set. STAB and psychic move, then choice of power or setup/wall stopping. Brine's best for finishing off that damaged pokemon.
Counters
Anything faster with Energy Ball/Thunderbolt. Alakazam destroys the thing, but the issue for Golduck countering is the lack of true hard counters, due to the diversity of its movepool.
Pros/Cons
+Decent typing, pure water.
+Massive movepool
+Gets a couple of good moves that can completely flip a battle
+It's evolved from Psyduck. That's a redeeming factor in itself.
-Stats aren't amazing...
-Slow. Outsped by pretty much everything else in competitive battling, and a few that aren't. Lilligant is a huge threat due to Petal Dance. Hell, Dedenne might be capable of beating a Golduck!
Conclusion
This guy has a niche. It should not be used outside of this niche in any tier above NU. In NU, it can try to sweep, but NU SS teams aren't too common, and that's how it can possibly sweep.
Subnote: This guy's best use is as a HM/field move slave ingame. It gets Strength, Flash, Surf and Waterfall, plus a couple of others in the older games.