I know that Water/Psychic is obvious, but that's why I'm for it; for too long, this improved version of Psyduck has been criminally non-Psychic. Water/Fighting takes it in a direction that seems more engineered for competitive mons than a natural extension of flavor (which, I mean, it is, but I still like a bit of subtlety). As to your point about moves, Golduck can learn 8 Fighting moves, none through level-up: Cross Chop, Dynamic Punch, Focus Blast, Focus Punch, Low Kick, Low Sweep, Rock Smash, Power Up Punch, Seismic Toss. Meanwhile it learns 14 Psychic moves, four through level-up: Amnesia, Confusion, Wonder Room, Zen Headbutt, Calm Mind, Future Sight, Light Screen, Hypnosis, Psybeam, Psychic, Psyshock, Role Play, Synchronoise, and Telekinesis (I'm not counting Rest given its ubiquity). Yes, it learns more Fighting attack moves, but what does that matter when you look at not only the amount of total moves per type, and Golduck's natural level-up moveset clearly favoring Psychic?You know that Mega Aggron gained 50 Defense? And it already has like the third highest Defense in the entire game...
Mega Charizard Y gained 50 SpA, Mega Heracross gained 60 Atk. These were already its best stats, meanwhile for Golduck it was one of its average stats (I mean they're all average, but still).
Making Golduck Water/Psychic is obvious, and really doesn't do it any favors. It also gets quite a few Fighting attacks (of particular interest are Cross Chop and Low Sweep), and is said to be "faster than even the most athletic swimmer" and "the fastest swimmer among all Pokémon". All it says about psychic powers are that it sometimes uses them, and it learns fewer Psychic-type attacks.
And 132 Atk is just barely better than Mega Charizard X, who has stronger STABs (Outrage and Flare Blitz) as well as Dragon Dance (Golduck only has Hone Claws, which is good with Cross Chop, but not as good). It's creative, viable, and still makes sense.
It's not like we have Speed Boost to Snorlax or something.
There's nothing I appreciate more in discussions like this than faithfulness with cleverness, rather than boosts for convenience's sake. The question shouldn't be "how do we make Golduck good", but "how do we make Golduck good in a way that's faithful to its premise"? It's far more interesting, I think, to work out a way to make Water/Psychic unique to our duck.
We've seen this typing in bulky mons (the Slow Bros) and speedy mons (Starmie), so what other routes can we take to set Golduck apart? Right off the bat, its best stat is Special Attack, and while the other three Water/Psychics out there have decent Special Attack stats themselves, none of them have it as their highest focus in terms of base stats. Moreover, Golduck's stats are localized, but low, rather than the Slows sacrificing speed and Starmie sacrificing bulk.
I'd say, if we want to match the spirit of the duck, we would continue this trend of well-rounded stats. For an ability, we could still use the likes of Tough Claws to allow us to boost Special Attack and Speed higher, but I think a far more fun solution would be to look back at Psyduck's influence on Golduck, bringing it back into zen mode. No, I'm not suggesting actual Zen Mode, but Simple. With Simple, Golduck can hit hard from either side, either with Calm Mind for great Special prowess, or Hone Claws for a physical set (not only acting like Swords Dance, but allowing it to use inaccurate moves like Aqua Tail, Zen Headbutt, and yes, Cross Chop, with impunity, or boast a perfectly accurate Hypnosis).
I'll let you have the last word on this one if you want to post a rebuttal, because I don't think this thread should be really deciding these things yet, but I wanted to give you a glimpse into my psyche and see precisely what I think of as a good extension of a mon that works through imperfect typing or stats, rather than what I see as a Mega purely of competitive convenience with contrived flavor.