There was the poor bear, spluttering away in the water, and trying his hardest to swim whilst the goldfish darted at him in fury.
What does to dart at mean in this context? Does it mean "attack"?
In this context, dart is Verb, it means "To move suddenly and rapidly", check here http://www.thefreedictionary.com/dart.
Those goldfish are moving rapidly at him. I think it's understandable