I believe it suggests that the water adjacent to the property is subject to raparian rights. Every riparian owner is entitled to the benefit of the water in its natural state. Whenever, therefore, the owner of land dams or impedes the water in such a manner as to back it on his neighbor above, he is liable to an action; for no one has a right to alter the level of the water, either where it enters, or where it leaves his property.
I can't see how this issue would relate to the suitability of the water for swimming. If something is creating backwater then that would infer that the water would be higher than it is in its natural state hence, a bit deeper for swimming. Check with the Realtor or owner for further clarification. Good luck!
Diane Wheatley, Broker