From The Heroes of Might and Magic III wiki

Class	Water Magic
Basic cost	5
Duration	1 round/Spell Power
Level	1st

Level	Effect
None	Damage from Water Magic spells is reduced by 30% for target,allied troop
Basic	Cost reduced by 1
Advanced	Damage reduction increased to 50%
Expert	Affects all allied troops

[edit] Units capable of casting Protection from Water

• Ice Elementals