"Saving A Life"
Maybe because Ainsley used her ability on the boat and the boat was essentially protected by the barrier, which was why the rules of the lake that prevented any visible ability to be used didn't react at all, or maybe because the barrier was considered 'invisible'.
In short, everyone on the boat could still use their various visible abilities and could even use abilities that were considered 'not in line' with the lake's ecology.
That split second was extremely short.
The plant-tamer girl didn't even realize that a sword phantom stabbed towards her chest because she was busy stretching her hand to take the Thumbelina Flower of the Man-eating Water Lily.
Just right when the girl grabbed the palm-sized flower, Ainsley's small barrier protected the girl's heart and the next millisecond, that sword phantom pierced through to the barrier.
PUFF.