WHY? WHY DOES EVERYTHING HAVE TO BE SO CUTE???
I'm honestly thinking about doing a "OMFG SO CUTE! I NEED TO CUDDLE IT!"-Playthrough
But yeah... since I've been officially challenged... give me a bit to think about it...
So, Rain Dish / Liquid Ooze...
Rain Dish is a Water-Type exlcusive Ability in the 'normal' games... while Liquid Ooze is Poison-Type exclusive
Hell Tentacool/Tentacruel are the only ones who have both, like this little guy... and they are Water/Poison...
The Sea Wasp is a DEADLY Jellyfish containing DEADLY Poison living in WATER (they are Jellyfish, duh)... So Water/Poison Typing would make the MOST sense!
BUT!
This wouldn't be Pokemon if everything would make sense! - Malamar is a Calamari... and no sign of Water-Typing
My guess would be that you want to leave a "false trace" with those abilities... and that those abilities should represent their respective types for this Pokemon, which it DOESN'T utilize or only half utilizes.
So even if it isn't a water and/or poison type, I would say those abilities and some moves it may learn (Poisonsting, Toxin, Aqua Jet come to mind) will give it coverage for those types... as you said from the beginning... most Pokemon aren't what they seem, and they have utilities to outsmart their weaknesses or have some things about them that makes them great.
So I would say this is one of those Pokemon and that Water/Poison is WAY to obvious for this little cutie... and with the case of Malamar, an Aquatic-Liveform in the Realworld doesn't neccesserily have to be a Water Type in Pokemon.
So... I'll scratch the Water-Type... but, since you said we wouldn't be able to catch it, that may because it lives in the water and we don't get a meaning of fishing! A non Water-type that lives in the water... it would suit you guy (and Ame's cruelty)
From the 4 Types you mentioned we have 3 left. Poison, Fairy and Psychic... I don't want to scratch poison yet... because Sea Wasps are like... all about their Poison. It's their main meanings of defense!
I don't think it's a Fairy/Psychic type... I can't point my finger on it... but it just doesn't lookt like it for me (and we already have a few original Fairy/Psychic type)...
so what's left for me is:
Poison/Fairy or Poison/Psychic
My guess would be one of those... but which... THAT IS the hard call...
Both pairings bring good resistences with them...
Poison/Fairy bringe more resistences with it, 5 resitences (double-resist: Bug and Fighting ; normal resist: Grass, Dark and Fairy ; and one immunity: Dragon)
it's weaknesses are: Psychic, Steel and Ground ... some considerably hard to come by early game types...
Poison/Psychic brings 4 resistances (double resists: Fighting ; normal resists: Grass, Poison and Fairy)
it's weaknesses are: Dark, Ghost and Ground .. also some hard to come by moves early game... well maybe not Dark because of the rather common "Bite"
I actually can't decide... the more I tend to one typing the more I'm inclined to say it's the other...