You need to pay attention to your loot. Read everything after a battle and make sure you can see your character, in this case when not near the lake when you read the item (scroll/book i don't remember) your character will say it had no effect (or similar). Anytime he says this it just means you are in the wrong place.
These sort of items usually cannot be sold or destroyed so it may be dropped or in a chest or just on a different character, maybe a companion/henchman you dismissed.
