Nagas are beings that live in the area where is water, for example, the rivers, lakes, streams, seas, etc. The nagas are often depicted having an upper body of a human and a lower body of a snake. Sometimes they appear in the form of fish, frog or snake.
When we go to places where there is water, we have to be careful of our action. I have heard of people peeing in the river and offended the Naga. They end up being sick and the doctor couldn't make the diagnosis. In some serious cases, the offender is possessed by the Naga.
Some of the signs that we might have offended the nagas are dreams of snakes attacking us, skin diseases, cancer, etc. If we think we are disturbed by the Nagas, it is advisable to seek help from a qualified Buddhist master. Therefore, when we go to areas where there is water, we have to be alert so we don't offend the Nagas.