A motorboat towing an individual or individuals on water skis, aquaplane, or other similar device shall keep at all times at least 100 feet from the shore, wharf, pier, piling, jetty, bridge structure or abutment, or an individual or individuals in the water. When passing another vessel, the operator of the towing vessel shall stay at least 100 feet away from the other vessel. Towlines may not be more than 75 feet in length, except, when barefoot skiing, a tow line 100 feet in length may be used. An individual or individuals may begin skiing from shore, boat deck, or wharf. On the slack water areas above Dams 3, 4, and 5 on the Upper Potomac River, a motorboat may not tow more than two tubes or similar devices when the tow line is attached directly to the device.