Reading your comments I believe you want one that is not post-game it seems?
Well, the only one that have then there is not dual type with any type you are already using and is from generation five is Simipour in that game. However if it could use post-game pokémons, pokémons from another generation that also have there or even dual types using the same types you are already using I would sujest something else.
But according to what you are asking, this pokémon is Simipour.
It can learn both surf and watterfall, as you want (however I would not advice, especially if you want to beat in hard mode)
As for the movesets:
Ability: Gluttony (Is the ability that you can have if you gonna catch it on the wild, make good use of this ability in places like battle subway is easy, just let it hold a berry with good effect for what you want, like for example a berry that increase special atk in a pinch, but in-game it could be a little difficult, but with the criteria you want, it has to be this, In-game let it hold the berries that can give it some support but you have acess, like preventing sleeping for example, as is easy to get)
It could be difficult to EV train in the in-game team early/mid game, but if you ever get some of the drinks that raise stats, focusing in special attack/speed.
For covering Partners
Ability: Gluttony (same case as before)
EV (same as above)
Ability: Gluttony (Lum/chesto/Cheri/Persim berry)
EV in Special defense/speed
I'l never advice to use the same pokémon that fights to do HM tasks, it limit you too much but if you really want do this. The following setup is not terrible for both things:
Nature: Rash (since increase spl attack but doesn't dreacrease the attack stats since will be needed)
Ability: Gluttony (same as The first two movesets)
EV in Special attack, attack, speed.
(Just a reminder, i'l never use this last moveset, i'm just trying to be nice and help you the way I believe you want, but the moveset who really would do greath would be the first three ones)
Hope I helped, have a good day!