The story
Referring to the emergence of predators such as sharks (and possible feeding frenzy) when blood is spilled in the water.
Etymology from Wiktionary, CC BY-SA 4.0.
blood in the water
Today it means: In a competitive situation, the exhibition of apparent weakness or vulnerability by one party, especially when this leads to a feeling of vulnerability or greater pressure to perform on the part of the weak party, or enhanced expectation of victory by the others. Full definition →