Non-stop at full speed a Citycat would take about an hour to go from the cruise terminal to the CBD.
They would not be allowed to use the ferry terminals, so they would have to find a public jetty to tie up to.
$8 seems optimistic as this is would not be public transport. There is the cost of hiring the Citycat, insurance, fuel, staff, etc.
At the moment it's a daydream and we just have to put up with a lack of public transport. Just as we do with almost all cruise terminals around the country that were not lucky enough to be built next to Circular Quay.