Sure do mate... as our girl loves swimming.
1. BEST ONE: Rowland Reserve, Bayview. Dedicated Dog Park and beach (all off leash). Can get busy at times. Owners are all lovely I've found and only seen one flare up there before. Sandbar goes out into the Pittwater inlet and the tide comes back towards the shoreline. Water stations and poo bags and bins. 24/7 access.
2. Yarramundi Reserve (North side) at Yaramundi (near Castlereagh) has direct access to the Hawkesbury/Nepean river. There's an off leash dog swimming area there too. Open during daylight hours but I've been there as late as 9pm and people haven't locked up.
3. Sirius Cove (next bay around from Taronga Zoo). Operates during weekdays only at specific hours though. Same with Chowder Bay. Been there a few times and it's a shallow cove with amenities block. Water comes back towards the shoreline. Off leash.
4. Spit Bridge Reserve they can swim at (not sure on times though).
5. Lake Parramatta technically isn't allowed but I've seen people do it.
6. Collaroy and Manly Lagoon apparently dogs can swim in, but water quality is questionable.
7. Silver Beach, Kurnell -
WOULD NOT RECOMMEND as a dog was taken by a shark there last year.
These few links are pretty good:
http://www.greatdogwalks.com.au/sydney-dog-friendly-beaches.aspx
https://www.goget.com.au/blog/dog-friendly-beaches-near-sydney/
http://postcardsydney.com/the-real-list-of-dog-friendly-beaches-in-sydney/
There's also a book at Dymocks about Australias best Dog spots. Covers everything from accommodation to beaches to parks, and things to do/eat.