As some people have said previously, some places are very seasonal- what may be great in summer could be total crap in winter.
At nearly all places in Australia, if there is a beach with water and wind, you will be right to sail.
As a general rule, north of Maryborough and Carnarvon (tropical areas), there are salt water crocodiles and box jellyfish, but it depends where you are. Box jellyfish are worst in the summer, but a full length lycra or wetsuit will minimise the risk of stings. There are still lots of places okay to swim/windsurf anyway, even in the tropical areas.
Apart from major population centres and tourist spots (eg Sydney Harbour) you won't have to worry about ferries etc.
As far as National Parks go, for some you will need to buy permits, but you can get them on a daily basis if needed when you go to the national park.
I think there are probably lots of great places to sail that are a bit off the radar- partly because they are not close to major population centres.
If you look on
www.bom.gov.au/climate/averages/wind/selection_map.shtml you can get a wind record for sites all around Australia.
According to these, Sydney should still be okay for sea breezes in Feb- March.
If you are travelling around it may just be a case of take it when it comes- along a lot of SE Australia in autumn and winter there is a fair chance of a 15-20 knot S or SE wind that comes through with a front for a day or two. Lots of good beaches, lakes, bays between Sydney and Adelaide for sailing.
The coast of north Queensland in winter should have a pretty good chance of 15-20 knot SE winds too.
Clarence