In his Recherches sur Us Superstitions en Chine, Pere Henri Dore, S.J., relates the legends he had heard with regard to this deity.  One of these is as follows: 

Shui-mu Niang-niang inundated the town of Ssu-chou almost every year.  A report was presented to Yu Huang, Lord of the Skies, begging him to put an end to the scourge which devastated the country and cost so many lives.  The Lord of the Skies commanded the Great Kings of the Skies and their generals to raise troops and take the field in order to capture this goddess and deprive her of the power of doing further mischief.  But her tricks triumphed over force, and the city continued to be periodically devastated by inundations.

One day Shui-mu Niang-niang was seen near the city gate carrying two buckets of water.  Li Lao-chuen suspected some plot, but, an open attack being too risky, he preferred to adopt a ruse.  He went and bought a donkey, led it to the buckets of water, and let it drink their contents.  Unfortunately the animal could not drink all the water, so that a little remained at the bottom of the buckets.  Now these magical buckets contained the sources of the five great lakes, which held enough water to inundate the whole of China.  Shui-mu Niang-niang with her foot overturned one of the buckets, and the water that had remained in it was enough to cause a formidable flood, which submerged the unfortunate town, and buried it for ever under the immense sheet of water called the Lake of Hung-tse.

So great a crime deserved an exemplary punishment, and accordingly Yue Huang sent reinforcements to his armies, and a pursuit of the goddess was methodically organized.

The Magic Vermicelli

Sun Hou-tzu, the Monkey Sun, [25] the rapid courier, who in a single skip could traverse 108,000 li (36,000 miles), started in pursuit and caught her up, but the astute goddess was clever enough to slip through his fingers.  Sun Hou-tzu, furious at this setback, went to ask Kuan-yin P’u-sa to come to his aid.  She promised to do so.  As one may imagine, the furious race she had had to escape from her enemy had given Shui-mu Niang-niang a good appetite.  Exhausted with fatigue, and with an empty stomach, she caught sight of a woman selling vermicelli, who had just prepared two bowls of it and was awaiting customers.  Shui-mu Niang-niang went up to her and began to eat the strength-giving food with avidity.  No sooner had she eaten half of the vermicelli than it changed in her stomach into iron chains, which wound round her intestines.  The end of the chain protruded from her mouth, and the contents of the bowl became another long chain which welded itself to the end which stuck out beyond her lips.  The vermicelli-seller was no other than Kuan-yin P’u-sa herself, who had conceived this stratagem as a means of ridding herself of this evil-working goddess.  She ordered Sun Hou-tzu to take her down a deep well at the foot of a mountain in Hsue-i Hsien and to fasten her securely there.  It is there that Shui-mu Niang-niang remains in her liquid prison.  The end of the chain is to be seen when the water is low.