If Buizel will only get a new moveset, you probably want to include an Ice-type move.
These are all Ice-type moves Buizel can learn:
Ice Beam (TM)
Icy Wind (Plat. MT)
Ice Punch (Plat. MT)
Personally, I don't see any of those used by Ash's Buizel.
Floatzel, however, gets Ice Fang on evolution, and it isn't a random TM or Move Tutor GameFreak decided to give Buizel just because he's a Water-type even though it doesn't really fit.
If he does evolve, I hope the writers realize he's the fastest Water-type (along with Starmie), and that he's even faster in water (Swift Swim).