There is a sailing event each year called the Darwin-Bali Rally (see
website) that departs late July and takes about 1 month to reach Bali and 3 months to reach Thailand. There might be boats sailing at other times but definitely not as frequently.
Many boats need crew (not necessarily experienced) so you can try approaching them directly from the race website or by registerıng on sites like
Boatıng Oz. My first boat fell through so I just turned up at Darwin marına a couple of weeks beforehand and asked around - people will be looking for crew up to a week out (as it takes about a week to do the paperwork).
There's plenty of description and pics from the 2005 event in the blog linked below, ıncluding a summary of boat life and what is expected of crew.
Any questıons after all of that, please ask.
TT
Oh yeah - none of the 70 beats last year encourtered any pirates. They're the least of your worrıes...