The main ramp for Pittwater is at Bayview. It's council run and has plenty of parking, as well as a wash bay and a jetty to load supplies on. Paying for parking is what allows councils to provide services such as this for users. Some might see it as expensive, but far cheaper than a hotel. The parking Sectorsteve spoke about is just on the street and really not that brilliant IMO.
Here is a link for all the local ramps to Pittwater.
www.northernbeaches.nsw.gov.au/ If you launch at Bayview, you're in the very South of the Broken Bay system. It will take under an hour to sail to West Head/Barrenjoey in easy conditions, or under motor at 5 knots. Between Bayview and West Head there are great bays to stay, including Towlers, Clareville and Coasters Retreat/The Basin. Once you head past West Head, if you turn East, you're in the ocean. Straight North past the East side of Lion Island and you make your way into Brisbane Water. There are some small towns including Woy Woy and some quiet bays to stay over in.
Turn West, after West Head and you make your way down towards Refuge Bay, Akuna Bay and Cottage Point. All in the Southern section of Broken Bay and all great places to stay and explore. In my Top Hat on a slow day I take about two hours from the Basin to Refuge Bay. If you turn North West between Juno Point and Eleanor Bluffs, you're in the Hawkebury River with Dangar Island dead ahead and Brooklyn on the Western shore. Turn West again between Dangar and Brooklyn and you go under the Rail Bridge.... 11 metres. Watch out for the current eddies. Then West through to the road bridge and the entry into the other side of Brooklyn township on the Southern shore.
For me, it's about another hour to an hour and a half to the freeway bridge from Refuge Bay. The Northern shore has the Deerubbin Park Boat ramp and car park on the Western side of the F1 freeway. I think it's free and at the moment, open, but there are no services and not sure if there is a jetty. Once past the freeway, there are some great stretches of water, and some restaurants as well like Pete's Bite and Berowra Waters Inn. Once into the main Hawkesbury system it's about 90km from the Rail Bridge to Windsor. Very quiet apart from ski boats, but there are supplies at Berowra, Spencer, Wisemans Ferry, Dargle, Sackville and Windsor.
Here also is a link to all the ramps near Brooklyn
www.rms.nsw.gov.au/maritime/using-waterways/boat-ramps-map/#/?address=brooklyn%2C%20NSW&radius=20000 I've used the ramp at Akuna for a powerboat and it's got a bit of a drop off that you can't usually pull up over if the boat is loaded. Empty it's fine. Facilities there are good and parking is usually easy, but the fee is $12 a day. Not bad really.
www.nationalparks.nsw.gov.au/things-to-do/Picnic-areas/Akuna-Bay there is a photo of the ramp in this pic. As Yara said, not sure how secure everything is there, but the National Park is technically closed from dusk til dawn, although they don't lock any gates.
Phone reception in the main passages is very good, on Telstra, but tucked away in small bays it can be patchy.
Hope you have a great trip!