Azumarill, Floatzel and Swanna all work well in-game. Huge Power Azumarill tears holes in things, Floatzel's fast and hits well enough, and Swanna makes up for mediocre offensive stats with high power moves and general convenience by being a joint Surf/Fly user. And naturally there's always Vaporeon. Lapras is in-game but tricky to get unlike the others due to low encounter rate close to end-game, but it's still a wrecker with access to STAB Ice Beam and being able to learn Thunder/bolt. You can also get Starmie, Walrein (albeit only in Winter), Wailord and Octillery around Route 13/Undella Bay, all of whom are decent Water mons.
If this were just B/W I'd stick with Samurott or Carracosta.
'Success rates are but scientific theory. A brave heart will defy any odds!' 'Don't beg for things, do it yourself or else you won't get anything.'