Or crocodiles.
As a deterrent, salt water crocs are what you want. They can live in fresh water habitats (like rivers). Larger & more aggressive than fresh water crocs ... eat larger prey, too.
From: https://www.wikihow.com/Tell-a-Freshwater-Crocodile-from-a-Saltwater-Crocodile
Saltwater crocodiles, or “salties,” will often eat small reptiles, fish, turtles, and wading birds. However, they are also prone to eating larger prey like wild pigs, buffaloes, and livestock like cattle and horses.
Freshwater crocodiles, or “freshies,” will eat smaller animals that are found near rivers, such as insects, fish, frogs, turtles, waterbirds, and snakes. Sometimes they will also eat small animals.
A big dietary difference is the consumption of people. Saltwater crocodiles have been known to attack and eat humans, while freshwater crocodiles do not usually attack people or eat people.