I've actually used Golduck before on rain teams and it isnt bad, but your post doesnt do it any justice. What Golduck has over other Swift Swimmers is a decently high special attack (base 95, which is the same as Kingdra, lower than Omastar, and higher than Ludicolo and Seismitoad) high speed (tied with Kingdra at base 85, which is fast for a swift swimmer) and a great movepool with surprise value. Keep in mind that although base 95 sp atk and base 85 speed arent great, Golduck is being used in the rain. So it's water stab gets a 1.5x boost and its speed is doubled, meaning that with 228 speed evs you can outspeed choice scarf latios, unless youd rather invest in bulk. Surprisingly enough, Golduck has more initial power than Kabutops and is tied with Kingdra for second due to Hydro Pump's high base power and is only behind omastar out of swift swimmers in the rain.
At this point, you might be thinking "even though Golduck has decent stats and is better than i thought, why would i use it over Omastar or Kingdra?" While Omastar and Kingdra are generally better, Golduck has a few important niches over both of them. Golduck's movepool is absolutely phenominal, despite not having a secondary stab. In a metagame where Ferrothorn, Mega Venusaur, and Amoongus are some of the most common road blocks to rain teams, Golduck has the right coverage to take them out. Focus Blast easily 2hkos Ferrothorn and Psychic 2hkos both Mega Venusaur and Amoongus. Sure, Omastar or Kingdra can probably get past these threats after a few switch ins (maybe not Amoongus though), but they need choice specs and good prediction (in Kingdra's case) to do this. Golduck, on the other hand, only needs a life orb to net these 2hkos and can usually 2hko most of the pokemon i mentioned with the combination of Hydro Pump and the appropriate coverage move, given theres a little bit of prior damage. Because of this, Golduck often doesnt even have to predict or resort to a choice item to beat these common threats to rain teams. Omastar has to continually spam Specs Hydro Pump to eventually wear them down (which wont work on Amoongus). Kingdra needs choice specs and good prediction if it wants a chance at taking down Mega Venusaur or Amoongus. Even with all of this, Draco Meteor drops Kingdra's sp atk and dragon pulse is piss weak. This means that even with a choice item and good prediction, Kingdra might barely get past Mega Venusaur, probably cant get past Amoongus (due to Regenerator) and cant get past Ferrothorn. Golduck can do all of this with a life orb and scores clean 2hkos, meaning that it wont have to keep switching out and back in to beat these roadblocks.
Now, you might be thinking "So Golduck fairs a little better against common stops to rain teams, but why not just use Tornadus-T or something else that can also reliably take them out?" To this I respond "If you sent in your Tornadus-T, your opponent would never send in Mega Venusaur, Amoongus, etc. However, if you sent in Golduck, your opponent probably would send in their Mega Venusaur, Amoongus, etc. and Golduck could easily take them out so another swift swimmer (such as Kingdra or Kabutops) could sweep." In this way, Golduck is a great lure for rain teams. It lures in annoying checks to rain and takes them out for other members or itself to sweep.
I wish I could stop talking about Golduck's movepool since this is taking me a while, but there are still more things it can do. Like many water types, Golduck get Ice Beam. Ice Beam obviously has great coverage with Golduck's water stab and can hit Lati@s and Dragonite hard, but it isnt necessary. Calm Mind is a decent option on Golduck as well. Golduck will often find chances to set up because its high power and speed forcing things out under rain. After one boost, Golduck can often clean teams up very well, especially if your opponent switched into their Ferrothorn or something that they think can take Golduck when in reality it cant while Golduck boosts. Neither Omastar nor Kingdra have access to Calm Mind. Finally, Golduck has the option to use Psyshock over Psychic, but only do this if your using choice specs on Golduck. With choice specs, Psyshock 2hkos assault vest Azumarill after stealth rock 100% of the time, which i think no other specially attacking Swift Swimmer can do. Also, Psyshock will net the necessary 2hkos against Amoongus and Mega Venusaur as long as you have choice specs, otherwise stick with Psychic.
If only Focus Punch and Swift Swim were compatible than Golduck could 2hko Chansey with a Rash nature.
Golduck @ Life Orb / Choice Specs
Ability: Swift Swim
EVs: 28 HP / 252 SpA / 228 Spe
Modest Nature
- Hydro Pump / Surf
- Psychic / Psyshock
- Focus Blast / Calm Mind
- Ice Beam / Calm Mind
For those too lazy to read all that was above, I will quickly explain the moveset. Psychic 2hkoes Mega Venusaur and Amoongus, Focus Blast 2hkos Ferrothorn, and Ice Beam has good coverage with water. Calm Mind is an option over Ice Beam and Focus Blast to attempt a sweep. Set up in something that Golduck can easily force out under rain with its high speed and power. Also, if youre using a Magnezone for Ferrothorn (or hate focus blast's accuracy) use calm mind over focus blast. Life Orb is almost always the superior choice over choice specs, however
if youre a prediction god with choice specs Psyshock 2hkos Assault Vest Azumarill 100% of the time after stealth rock and the extra power from choice specs still allows Golduck to 2hko Mega Venusaur and Amoongus. Hydro Pump and Surf come down to personal preference. Never use scald because the point of scald is to burn checks and counters like Mega Venusaur, Amoongus, Ferrothorn, and the Lati@s, which Golduck beats anyways due to its coverage moves.
If Seismitoad deserves D for being immune to Thunder Wave, than Golduck deserves D as well.