lit. “let her not drink the waters (i.e. ‘drown’)”
I could see this working, but wouldn’t it make more sense to use a passive infinitive (obrui)? This ‘infinitive of purpose’ is very rare in classical Latin however. Negative imperatives generally use noli + infinitive. If I interpret this correctly, you are saying “do not lose her to be ruined”.
That is a good translation. I would put in the present tense though… ea ne demergatur. The person who gave you that phrase put it into the perfect (past) tense, which is usually used for a different sort of command, though still correct.
Unless you use the perfect participle it is impossible to show that the subject is feminine otherwise, though on second thought I would rather use illa than ea.