A Maw is a creature that lives on Cancer, in the depths of the Cancer sea. Their poison is deadly, unless treated quickly. A Maw is never supposed to come out of the depths of the ocean, so the only Cancerians to usually see them are ones that are deep diving.
Stanton Grace encounters one while swimming in the shallows, and gets attacked. His mother luckily saves him, and fends off the Maw.