Water + Arcane covers the weakness of Arcane's heavy hitters very well.
Wizard's Tower, Acid Ball, Dispel, Dissolve, Surging Wave and Teleport are all in school for Water Wizards. Liberal application of those can overcome the weaknesses of Hydras or Jellies very powerfully. And since those are all great spells the Wizard was going to include and cast anyway....
As for Druid, Wizard's Tower + Flameblast or Fireball is a hard pill to swallow. If Wizard wants to win that matchup badly enough (and is willing to invest the points) there's not much Druid can do to stop him, other than giving up on casting plants. The Wizard just needs be sure to bring more Towers than the Druid brings Force Hammers.
Druid's hope in this matchup is that the Wizard didn't bring many Fire spells because he had bigger fish to fry.