Should I swim in Brisbane river? Is the water quality is good enough for human to swim? I would like to ask someone who is familiar with the ecology situation around the Brisbane River? I heard a lot about pesticides dumped into it, so is it safe to swim? And what about shark attacks? ))

Is "It can not doesn't work" a correct phrase? Does the phrase mean, that it would work for sure? I mean to say that it would definitely work. How can I do that using double negative form? It can non not work, or It can not doesn't work, or else?