Complex sentence: Apart from this one exception, mariners remained completely without the navigational aid of either lights or buoys along the Huron's Canadian shore and the entirety of Georgian Bay until 1851, when range lights were constructed at Owen Sound Harbour.
Split into simpler sentences: Apart from this one light station, mariners remained completely without the navigational aid of either lights or buoys along Lake Huron's Canadian shore as well as the entirety of Georgian Bay. It was not until 1851, that a series of range lights were constructed at Owen Sound Harbour.