You may jump in but can you get out? Often people can’t get out of the water due to steep slimy banks or sides.
There is no way of knowing what lies under the surface of the water. There could be shopping trolleys, opened tin cans or broken bottles to mention a few.
If the water is polluted, it could make you seriously ill! For example, Weil’s disease is a form of infection that can be caught through contact with contaminated fresh water.