The Immigration Advice & Rights Centre has a pdf document on Visitor Visas, including the 651.
Essentially, you have to be able to prove to the immigration officer that you have the intent to return home. ONE of the means of doing so includes having a purchased return ticket, showing that you're leaving the country on day x.
Most countries have this requirement for tourist visas, and for example, in America, it's very common among travellers to purchase a cheap bus ticket from Seattle to Vancouver as 'evidence' that they're leaving. Then when they've worked out where they're actually going, they buy a ticket to there, and forfeit the bus ticket.
Leaving by sail boat would be harder to prove, and indeed what might be easier is to produce an 'onward' ticket - for example, a cheap flight to New Zealand, or Singapore or something. Ideally you'd purchase a refundable or adjustable ticket, so that you can change it or get a refund when/if you go by boat instead.
I recommend having a read of that pdf, it gives you several of the other options on what would be 'evidence' of intent to return, including a job, lease agreement or more.