The only places that no swimming is posted is at the ramps or at dams and it also says no fishing. This is a safety issue for a dedicated resource(the ramp is there for the specific purpose of launching boats) The only other area this happens at is a safety zone near dams. Nothing is absolute but in the open waters swimmers do have right of way. Same as a pedestrian in the middle of the street may be jay walking but it is a drivers responsibility to avoid the pedestrian. The only time the motorist is off the hook is when an accident is unavoidable as in a pedestrian suddenly jumping out from between to parked cars.
Plus you will find lakes and ponds (other than Lake PLeasant) that don't allow powerboats or limit them to electric motors while still being open to swimmers. The only waters I can think of that allow boats and not swimmers are some of the community lakes that don't allow swimmers for health and safety reasons. This is because the water quality is not maintained or it has drains that are not safe for swimmers. Plus these are considered private property and not open to public use. The case with tempe town lake is water quality there is generally unheathly and because it is really a wash area it is unsafe for general swimming so that is reserved for special events.
Plus you will find lakes and ponds (other than Lake PLeasant) that don't allow powerboats or limit them to electric motors while still being open to swimmers. The only waters I can think of that allow boats and not swimmers are some of the community lakes that don't allow swimmers for health and safety reasons. This is because the water quality is not maintained or it has drains that are not safe for swimmers. Plus these are considered private property and not open to public use. The case with tempe town lake is water quality there is generally unheathly and because it is really a wash area it is unsafe for general swimming so that is reserved for special events.