There are a few main ways that the Nile River was a source of danger to ancient Egyptians. First of all, the Nile is prone to seasonal flooding. It is true that this flooding is actually very beneficial as it refreshes the sediment necessary to agriculture. However, on occasion, the flooding could be larger than expected or occur at unexpected times. When this happened, work camps and villages in and near the floodplains could be suddenly swept away leading to many drownings. Conversely, the flooding may sometimes have been smaller than expected leading to droughts and famine.
Another danger that the ancient Egyptians were unaware of was that the Nile River was the home to many disease carrying parasites. After the seasonal floods, standing water in the fields acted as a nursery for malaria carrying mosquitos. This disease can lead to a debilitating fever and even death. While less deadly, parasites carrying schistosomiasis also lived in the water of the Nile. Schistosomiasis causes extremely uncomfortable and itchy rashes that would make it very difficult to work. If the infection is really bad, it can even cause damage to internal organs.
Larger dangerous animals should also be mentioned. The Nile River was home to countless hippopotamuses and crocodiles. If a worker unwittingly ventured too close to these large-toothed and dangerous creatures, they might have found themselves in a very hazardous and potentially deadly situation. In fact, the Nile crocodile and the hippopotamuses are regularly depicted in ancient Egyptian art.