My guess is that you are either trying to cast Soothing Waters as one of your NON CORE abilities or you are trying to attack AND cast Soothing Waters.
If you have the skill as one of your CORE attacks/abilities, it costs 3 AP to cast.
If you have the skill as one of your NON CORE attacks/abilities, it costs 4 AP to cast.
You only get 3 action points per round, so Soothing Waters needs to be your ONLY action for that round AND it needs to be one of your core actions. There may be some perk, ability, or item that gives you extra AP or reduces the amount of AP skills require, but I, personally, am unaware of anything that does that.
The other thing you you need, is to have the available Aura to cast it. It costs ~90 aura (with no spell efficiency bonus) to cast and if you do not have the Fire Spells perk, you cannot cast a spell that you don't have the available aura for.
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