*Spoiler alert* About the ending.

She didn't created an imaginary man. When she walked into the church she started to masturbate on the Jesus' image. It was some sort of sick immaculate conception. I guess, she didn't want to do it, because she moaned indignantly, but she did it anyway. Then the famous subway scene started. There, in a subway, she miscarried a horrible tentacled monster. This monster is the symbol of her lust and closed desires, she called it 'sister Faith', because her faith is perverted and gross. Since then she had been 'bringing it up' by feeding with parts of the detectives' bodies and by having sex with it. It became anthropomorphic at first, but gradually it became an ideal version of her husband - Anna had been in love with him although she betrayed him and screamed at him all the time. She wanted him to be perfect, to be more sexually satisfying, but it was impossible so she was angry at him. But the making of It saved her from frustration. Finally, she got what she wanted to.
Anyway, the whole movie is a metaphor of divorce. And both doppelgangers (Helen and unnamed Anna's creature) were left with Bob as a new inerrant version of his real parents. That means that real Anna and Mark made up for their son, but it wasn't sincerely. Children always feel things like that. Bob preferred to drown himself than to see his new 'father' and 'mother'.