How Do You Get To Singapore From Sydney By Boat?

You could charter a boat.  Find someone who is willing to ferry you.  You'd have to ask locally at travel agents and marinas.  

You also might be able to find a deal where you work for your passage on a commerical vessel.  Previous employment on boats not always required.  Expect some very long working days although if you have the right attitude, it can be a lot of fun, too.

One of the pleasant alternatives would be to catch one of the long-distance cruises.

Cunard runs 14- and 28- day cruises between the two cities.  

View 14-day itinerary, which just goes from Singapore to skirt around the west and southern coasts of Australia.

View 28-day itinerary, which takes in Vietnam, China and Japan, too.

Similarly, P&O offer 11- and 21-night cruises to get from Sydney to Singapore.