A natural harbour is a landform where part of a body of water is protected and deep enough to provide an anchorage. Many of these harbours are rias, a type of estuary at the mouth of a river where a valley has flooded. 

Natural harbours have been of great strategic, naval and economic importance and have therefore become the locations for many great cities of the world. Having a naturally protected harbour reduces or eliminates the need for breakwaters and will result in calmer conditions inside the harbour itself.

Sydney Harbour (Port Jackson) is thought by many to be the deepest and largest natural harbour in the world being over 11 miles long (17.7 km) and covering an area of 21 square miles (54 sq. km). The harbour contains several islands and is home to over 580 species of fish. The seabed is irregular and complex with some very deep holes up to 45m deep and shoals where the water depth can be less than 3m. 

Not only is it a beautiful waterway and a natural playground for sailing, water sports, swimming, diving and walking, but it is surrounded by hundreds of kilometres of shoreline, with national parks and historic sites, including the iconic opera house and Sydney harbour bridge,  not to mention Australia’s most famous city itself. 

According to theyachtmarket.com