Venice's Pescheria (pronounced "Peskaria"), a.k.a. the Rialto Fish Market, is a popular tourist attraction, but it's also a serious food market where you'll find restaurant chefs and homemakers selecting from an array of freshly-caught fish, crustaceans, and other seafood every morning but Monday.
Along with the Erberia, or produce market, the Pescheria is alongside the Grand Canal, just north of the Rialto Bridge as you exit the bridge across the canal from the Rialto vaporetto platforms.
The video below shows a typical morning at the fish market. For more information about the Pescheria and Erberia (including how to find them), see our Rialto Food Markets article at Venice for Visitors.