Anaconda's spend a lot of time in Water which gives them the name "Water Boa". Many of the water mammals and other species that live around the water bodies, form the prey species. They can eat anything they can overpower & kill including fish, birds, mammals like Tapir, Capybara, Reptiles like Caiman etc. They constrict (squeeze) their prey to kill and then swallow them whole. They usually prefer prey that they can swallow. Large prey like a Buffalo or an Adult Human are impossible to swallow for the Anaconda. So, they don't usually eat them. However, they can easily kill any living animal.