From memory there are a number of marinas on the Hawkesbury River and in Pittwater.
A quick search on the internet turned up:
http://www.brooklynmarina.com.au/ http://www.dalboramarinas.com.au/bays/akuna-fac.html
I'm sure that there are many more.
There are also many parks on Pittwater, Broken Bay and the Hawesbury river that will have toilets and possibly showers.
Some of the waterfront hotels will also have facilities.
The area is very popular for just the activity that you have in mind so I am sure that you will have no wories.