Need advice on a letter to my boss' boss.

I work at a municipal pool. My manager has a nasty habit of being in a bad mood, nearly every day. I feel that no matter what her personal life is, she should not take it out on other employees. She is a control freak on a power trip. I am 28 years old, and she treats me like a teenager. She treats the lifeguards like kindergartners. She has exhibited behaviors such as favoritism, prejudice, outbursts of anger, making children cry, etc. She has even gone so far as to tell a few of us that we "need to get saved". She has also told me "I gave you every Sunday off, because you need to go to church." This is somewhat disturbing to me. She is unapproachable with concerns or problems. There are families and children that will not come to the pool when she is there. I could go on and on about things that she has said or done that are inappropriate. There are 6 lifeguards that are writing letters to the city, and numerous parents as well. The goal of these letters will be to ensure that she is not hired for this position next year. I am somewhat scared of being retaliated against if she finds out that I am writing a letter. Would you write the letter, or does it seem inappropriate to go over her head? Have you written such a letter? What details should I include?