Well the water type that is actually considered the best in Unova, is Jellicent.
It's a super useful pokemon with very unique typing. But in my opinion, it's one of the best water types in Unova. ( And competitively speaking, it's in a higher tier than all of the other water types from Unova. Even Keldeo )
So before I tell you where to get it, I'm also going to recommend two movesets for it.
#1 will be for in-game use ( playing through the game )
#2 will be for if you decide to use it in any kind of competitive play
Set # 1
Surf - STAB and having the HM on your team ( HM )
Shadow Ball / Ominous Wind - Shadow Ball is better, but Ominous wind is easier to access. Either way, it's an STAB slot. ( TM / LV 27 )
Recover - Saves you some cash on potions / super potion / hyper potions. ( LV 17 )
Energy Ball / Psychic - Coverage! ( TM / TM )
Set # 2
EVs: 252 HP / 252 Sp DEF / 4 Sp ATK
Scald - STAB and burn
Hex - Slightly gimmicky, but deals good damage after the burn kicks in
Recover - Recovery is amazing in competitive
Energy Ball - Good coverage.
You can find it's pre-evolved form by surfing at..
Route 4, 17, 18, Driftveil City, and P2 Laboratory
For Jellicent itself:
Surfing in Route 4, 17, 18, Driftveil City, and P2 Laboratory ( Same places as it's pre-evo )