Oops, some more Googling brought me to the SOS website which had these directions:
"The Rothesay, is located in the St. Lawrence River, west of Prescott. It is accessible both by land and by boat. Follow Highway 401 and take the Highway 18 Exit (#716) to Prescott. Turn west on King St. W (Highway 2) and follow for 1.75 Km until you reach Riverview Heights. There you will see a Historic Marker along the south side of the road commemorating Justus Sherwood. The adjoining grass strip can accommodate about a dozen cars. From the embankment you will see a buoy approximately 300 ft out, marking the actual wreck and a jug closer in that is the beginning of the line."