Conister Rock

St Mary's Isle is a partially submerged reef in Douglas Bay on the Isle of Man. Prior to 1832 the rock was the property of the Quane family until John Quane, Attorney General of the Isle of Man, presented the rock to Sir William Hillary, in his capacity as President of the Isle of Man District of the Royal National Lifeboat Institution to build the Tower of Refuge. Since then the custody of St Mary's Isle has been held by each president of the local lifeboat association.

The reef is not far from the ferry terminal. Its eastern part, the Conister Shoals, had to be blasted away when a new harbour breakwater was built in the early 1980s. The rocks are only fully submerged at spring high tides; they can be accessed very briefly on foot at low spring tides.