NSW Maritime has released an update on boating rules as they apply during the current lockdown of Greater Sydney.
The full rules can be found here.
Key points:

• Water police are enforcing the 10km radius rule from your home to where you are boating. The police are using a website called 2kmfromhome.com to determine whether or not your home address is within the 10km radius. You can use it yourself - go to that address, click "search", then type in your home address.
  On the navigation bar you can select a distance and the map will then show a radius - if your boat usage area falls within that radius then you may be able to use your boat for a permitted purpose (see below)
• Note re above - the radius rule applies to wherever you are when the police pull you over. So if your mooring or boat ramp is just inside the radius but then you move to a point further away, you will have breached the 10km rule.
• There are a number of permitted reasons to be on a boat - see the media release for full details. In general terms they are:
  • Exercise (check how it is defined) - note that recreational power boating or cruising is not deemed to be 'exercise'
  • Fishing
  • To maintain or repair your boat in line with your obligations to keep the vessel seaworthy
  • To move it for repair or relocation