It is not a common idiom. Even a native speaker has to glean the meaning from the context. This context suggests that "I swam dirty waters" refers to the same activities referenced in "please forgive me for my sins": swimming in dirty waters means living in dirty, or sinful, situations-- doing dirty (unclean, impure) things. The following line "but you pushed me in," then, means that the person she is addressing pushed her into those situations-- that is, his behavior forced her to act the way she did. (A common English idiom for this is "you drove me to do it.") Hope that helps!