Since you said you live around Hillary's I was going to provide a sarcastic answer of 'Look out your window', but I guess you questions is more about where is a good place to ride.
Check out jetsportwest.asn.au for details on the PWC club in the Swan River.
Towing. JetSkis are allowed in All (Most) Waterski areas as long as they are towing (or transiting to tow). Read the sign's available in the area if you are not sure, or look at the DPI website. No JetSki FreeStyle riding is allowed in WaterSki area's. Follow the same circum navigation the ski area uses, and stay far enough from shore.
Pinnaroo Pt (North of Sorrento Quay) is a designed Water Ski area. If Marin and Harbours catch you towing a biscuit, skier, or tube between Ocean Reef, and Sorrento, they will direct you to Pinnaroo Pt.
In the river, Goodman pt behind Burswood, or Deep Water Pt in Applecross.
Rottnest Island is NOT allowed. There are exceptions, but for now, lets stick to it being out of bounds. Too far off shore according to Marine Regulations.
Lots of us Northies drop the ski's in at Ocean Reef marina becuase it has a couple of nice beaches to moor at while getting ready for a long ride, or having fun in/around the area. But Sorrento or Mindarie are good to launch from too.
Plenty of fun ocean fun to be had between Sorrento Quay and Two Rocks. Quite often, people ride from Ocean Reef to the Alkimos Wreck for a nice cruise, and some wave jumping.
Wave jumping north of Ocean Reef, or Just South of Mindarie Keys. Keep away from the Surfers though. Plus Alkimos has some really great swell.
Little Island (aka Seal Island) off Pinnaroo Pt (Nth Sorrento Quay) is a nice trip, but need the PDF1 for that distance. No Seals all that often during summer.
SORE (South Of River Enduro) crew people ride from Rocko, and the Rocko PWC point, or the place Bent St Jetty. There is 'Crystal's for Wave Jumping, or HMAS Stirling to circum navigate, but stay away from the Naval base. Watch out for long rides from Rocko to Mandurah or further (like Bunbury).
River Races launch at the Narrows (JetSportWest) on Spring/Summer/Autumn race days, and social events, plus a few curises up/down the river, and if your are lucky enough, a sponsored ride to Rotto.
Have fun. Don't be shy in talking to other PWC people, we don't bite too hard.