Like all sensible hags, she takes her payment before she tells or gives you anyhting, so, unfortunately not. However, you never need to accept her deal. Just don't say yes; tell her it's a bad deal.
If you want to rescue Marina, you're going to have to cross Ethel and defeat her anyway - you don't need to forcefully attack her outside of story, if you'd rather not; you can just infiltrate her lair and let the events play out as they will beyond that point.
