Rose Bay isn't that big but yes probably bigger than the river. BUT in Rose Bay they have three sailing clubs, two stand up paddle clubs, two jet boat operators, Sydney ferries every 10 minutes, hire boats, party boats, canoe's and the Westfield parking lot all surrounded by some of the most expensive real estate in Australia.
I don't know how shallow the other parts of the river are but a Van only needs four foot of water to operate off the step and about two feet when on the step.