The thing you have to remember about natural bodies of water is that there is an exchange of water on a regular basis. There is a flow of water into and out of lakes, ponds, streams, and rivers from rain run off, underground springs, snow melt, ect. The point is they are not stagnant bodies of water where bacteria can survive and be transmitted easily to swimmers. Swimming pools are not lakes, they do not have natural water exchange and they do not have plants, fish, and other aquatic life that feed on harmful organisms in the water. Also in times of drought when natural water exchange is low lakes have been closed due to high bacteria levels. This can also occur in flood situations where too much water has been exchanged and there is not enough beneficial bacteria to kill off the onslaught of harmful bacteria that has washed into the natural bodies of water. Algae itself is not harmful, other than decreasing visibility so you would not see a swimmer under the water. All it takes is one swimmer with a recent case of loose bowels to contaminate a whole pool. Swimmers tend to swallow small amounts of water while swimming. Even if you are just standing knee deep can you guaranty someone is not going to splash water in your eyes or mucus membranes? Is algae itself harmful? No, but is a big red (or green!) flag that the water is not sanitary.
