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1.
"sharks in the water" refers to the other women in a relationship that a girl knows are there.
-"Why're you leaving him? You guys were so good together."
-"He thinks I'm too stupid to realize there's a shark in the water, I know he's going over some other girls place."
by crgouhsnf July 11, 2011
 
2.
Shark in the Water is a term used to convey that one is menstruating, and thus is dangerous; or to say that the "waters" are "off limits" for that week.
Not tonight, baby, there's a shark in the water.

You had better clean up after yourself, there's a shark in the water.
by Richard Sherpa March 27, 2011