In some game systems, spells and items will have special effects against certain creatures - more damage against evil beings, or undead, or living things, or golems and robots, or . . . whatever.
In GURPS, the standard is that creatures are all equally affected by the effect, unless they have a special Weakness or Vulnerability. In other words, fire doesn't do extra damage to "dry" beings, but requires that the "dry" creatures have a disadvantage that makes them more vulnerable. Holy water is just water to everyone - even unholy creatures - unless they're specifically written up to be weak to holy water or holy objects.
For example, the Sunlight spell does exactly what the name implies - it creates sunlight. That sunlight is perfectly ordinary - it doesn't do special damage to undead, or evil creatures, or creatures of shadow, or anything like that. But creatues such as vampires, or demons, or shadow creatures may have a Weakness that causes them to take damage from sunlight. Against them, the spell has an effect - but really, it's an effect of the Weakness, not the spell.
This is a small point, but it's a critical one to note. Don't expect your spell, item, or power to do anything special, unless your target is especially vulnerable or weak against it!