Most boat hire in Sydney puts you in the middle of the busiest water in the country, dodging ferries between Circular Quay and Darling Harbour. Leaving from Mosman gives you the opposite. Middle Harbour opens up immediately — wooded coves, sheltered bays, hardly any commercial traffic.
It also means you can choose. Turn left and you are in the quiet water of Bantry Bay and Sugarloaf Bay within twenty minutes. Turn right and you are out past Grotto Point with the whole harbour, the Opera House and the Bridge in front of you.
For anyone who has not driven a boat before, that first stretch of protected water is worth a great deal. You get to find your feet somewhere calm before deciding whether to go further.