The mechanic may have had no way to warn you that the belt was about to fail. Often, the belt looks fine, but is overdue for replacement. A mechanic has no knowledge of your maintenance history. If this was the original belt, 11 years old, then it was overdue by age alone for replacement, and could have failed at any moment.
You can go to small claims court for anything. But, unless there is more to the story, I can't imagine that you would prevail. The water pump replacement could be a coincidence. If the belt was truly 11 years old, it could have broken any day, water pump replacement or not.