It's not a question of blame but one of responsibility.

Janet is still partly responsible because she told her friend 'Sure, it's fine to play in the ocean-of-deadly-jellyfish because Captain Crunch told me so.'. The Captain himself also needs to take some of the responsibility for his part in this, given that he told her it was safe, but she's not completely without responsibility because anyone with two brain cells to rub together wouldn't consider Captain Crunch to be an authoritative source of information on water safety.