system
Given the following post, respond as if you're commenting on Reddit:
human
CMV: Psyduck should be a dual Water/Psychic type Pokemon

First of all, the name psyduck is a combination of the words "psychic" and "duck."  It's own name says that it's psychic.  That seems like a pretty good reason to be a psychic type pokemon.

Secondly, it learns as many psychic type moves as water type moves.  In its original form in generation I, it learns as many psychic type moves (confusion) as water type moves (hydro pump.)  Not only that, but it learns the psychic type move at level 36, a full 16 levels before it learns a water type move.

Third, its original pokedex entry reads "While lulling its enemies with its vacant look, this wily Pokémon will use psychokinetic powers. "  This clearly makes more references to its psychicness than its waterness.  In fact, none of its pokedex entries in any generation make reference to water or water moves.
gpt
In Gen 1, a bunch of things got messed up. 

Just check out Butterfree / Venomoth and compare them to Metapod / Venonat. 

Its pretty obvious that the art for Butterfree ended up on Venomoth and vice versa - based on what they evolved from. 

Similarly, the names of Psyduck/Golduck are obviously flipped. Psyduck is Golden while Golduck is Blue? Also, Canonicially, Psyduck doesn't yet realize it has psychic powers until it evolves to Golduck. While Psyduck can use psychic powers, it cannot do so purposefully or intentionally until it evolves (this is much more evident in the anime or manga, than in the games) - hence the typing. 

Finally, from a game balance perspective - slowbro and starmie are both already water/psychic - and having 3 different water/psychics in only 151 original gen 1 pokemon - may have seemed like too many.