Anacondas, like most snakes, are opportunistic feeders. They're also like most predators and would rather take an easy-to-catch meal than go to extra work for one. Monkeys are generally hard to catch, and too small to be much of a meal for a large anaconda. Not really worth the effort. Seems to me they'd prefer to take things on the immediate shoreline or already in the water - like capybara and caiman. Not to say that they wouldn't grab a monkey if they had the chance - not likely that it would jump from the water to the trees though.
They range through much of South America, mainly in and around high vegetation areas of rain forests of the Amazon and Orinoco basins and their tributaries.