By boat transport, I assume you mean ocean liner? There are a number of great ships going through Sydney on their travels.
Cunards new Queen Victoria would be my pick, but depends on your budget.
There is a good website for cruises is
www.vacationstogo.com - they have discounts shown.
Using this website it is really easy to search for different cruises. I have used them before and found their service great.
Looks like Cunard will be doing a 34 night Sydney to Los Angeles cruise on the Queen Elizabeth II starting February 25.
Also Princess Cruises has one leaving April 14 for 31 days aboard the Sapphire Princess from Sydney to Los Angeles.
Hope this helps. have a great time!
Links of interest:
Australia Cruises Things To Do in Australia Sydney Hostels