Ideas and suggestions.

It would be a logical (and atmospheric) enhancement to the offensive magic of water to add a slowing effect to the Ice Bolt and Ring of Cold spells (e.g., 1 round for advanced water magic and 2 rounds for expert water magic). However, simply adding a slowing effect for a number of rounds equal to the spell level wouldn't be overpowered; it would be a reasonable enhancement. In many games, when you cast a cold spell, it slows down opponents in addition to dealing damage.