I've noticed it happening with water elemental summons, how annoying it is to see your ally lose half of his hitpoints in an attempt to kill a skeleton or something equally weak

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I've played a druid in NWN before, but not up to 30th level, just through the main campaign I think.
I suppose the saving throw required against drowning increase with the power of the elemental. E.g. higher throws required with Huge Water Elementals than normal ones.
Air Elementals either knockdown or stun their enemies.