Hi Pantsman,
I was in Sydney two years ago and wanted to have a special meal on one particular night. I booked a Captain Cook Harbour cruise and, to be honest, thought in my ignorance that it would be a small affair on a decent sized boat circling the harbour.
I didn't know when I booked that the Harbour has a 27 mile coastline and that the "cruise" would be exactly that. The meal had seven courses and all wine and other drinks were in the price. There are various options when booking and I went for, if I remember correctly, the Gold Star package. This meant we were in the first group to board and were served Champagne with canapes on the top deck while all other groups were boarding.
It leaves from Circular Quay, so views as we sailed around the harbour were fantastic, starting obviously with the Bridge and Opera House, but there was lots to see up top at any time in the trip.
I would certainly recommend checking it out as an option. Here is a link to the relevant page on their website:
http://www.captaincook.com.au/home.asp?pageid=1661F5DD27C61DDC&destinationid=55F0AB3672931FB9&cruiseid=50A623E2ECCD4742
It's priced inside the budget you quoted, so you would have some cash left over for drinks in town when you get back on land.
Hope this helps,
JJ.