For a NU Manaphy. With Damp Rock Rain Dance, it can spam Rest easily, as Hydration will heal its status. Not much else to say.
For a nice, bulky water-type Pokemon with a cool ability. He can serve many nice roles with a good movepool, such nas Curse or Stockpile.
For a powerful Sheer Force Life Orb sweeper with access to Dragon Dance. Be afraid of it.
So overall, pick Quagsire for a bulky water-type that has lots of niches, or Feraligatr for an immensely powerful sweeper. But no Dewgong.