I think it would be okay for B to have two functions, as it does in the four classic games (In SMB2, B also serves to pick up and throw items). In Sunshine's case, tapping B would shoot water and holding B would make Mario run. I can imagine using FLUDD to hover by holding B in the air...Or hold the A button a la Peach in SMB2, since it's an almost identical mechanic.
Then that leaves holding/throwing/kicking items...
Then that leaves holding/throwing/kicking items...