Given my love for sea-otter Pokemon - both Dewott (or any other of his evolutions) and Buizel/Floatzel cannot be missing from my team [except for my all cat team in Black] or otherwise the game isn't fun at all. Two water types in my game, it makes a real challenge to come up a with a good team for gyms/E4/Champion.
Both Pokemon must know Surf and Aqua Tail, I always teach Ice Punch to Buizel, and Ice Beam to Dewott, the last attack may differ between Pokemon.
One holds Splash Plate, the other will hold Mystic Water