In the Mishna, Sotah 1:3 and 3:3 both speak of instances in which a woman suspected of having committed adultery admits of her crime before drinking the water. Given that the penalty for adultery is death (eg: Leviticus 20:10, Sanhedrin 11:1, etc), and given that the curse of the bitter waters might not take effect immediately (cf: Sotah 3:4), under what circumstances would a guilty woman confess to her crime? Since there are no witnesses, would not every woman choose to take her chances with the water?
if she confesses she won't be put to death by Beis Din since you need 2 witnesses for that.
and if there are 2 witnesses then she won't be tested by the sota water, hence there are not 2 witnesses.
therefore, she would be saving her life from the water by confessing and would not be executed by beis din.