The two most common monkeys that I saw everywhere along the river and its tributaries were the White-fronted Capuchin and the Red Howler Monkey. The capuchins were literally everywhere I looked and I saw them feeding, playing, fighting with other monkeys and often getting very close to curiously stare at me and then run away back into the trees.
The Howler monkeys were less curious in general and did not come as close. I could hear them from very far away as they make a loud noise using their large throats that they use to communicate with each other. Unlike the capuchins that live in large groups of 20 or more the Howler monkeys live in much smaller groups and were very territorial of the area they were foraging in. The howler monkeys ignored me for the most part except one day towards the end of my trip it started to rain hard and a group of 4 all huddled together under a very tall palm tree. I was lying on my back in the bottom of the canoe with my lens pointed straight up snapping away. There were three younger ones and they could all obviously see me and they would hide on a tree branch then quickly poke their heads around to stare down at me then back to hiding. It was the perfect time of year for monkey watching because the fruit and trees were abundant and they came right to the rivers edge to feed and play.
Beautiful animals!
It can be dangerous lurking directly below monkeys but it sounds like you didn't encounter any "dropping" debris.