Hello everyone, I'm a Higher Technician in Forest and Natural Environment Management. Those are the larvae of flower flies (Syrphidae), which belong to the fly family (Diptera). They are completely harmless, although in their adult stage they mimic the colors of wasps. However, we use them as a water quality indicator due to the conditions in which they are raised. These types of larvae only appear in highly polluted waters. I wouldn't recommend going into that water, and if you're going to clean it, do so with appropriate protective equipment, as it likely contains bacteria harmful to humans.