Ships are professional workplaces where qualified people are doing work for pay. I don't believe you'll get a ride with any such ship except as a paying passenger. For that, talk to a travel agent.
Yachts are probably more likely where you may be successful and the best place to start is in Panama City. I don't believe there are a lot of private vessels leaving from any other part of South America for Australasia. I haven't researched that, it's just my gut feel.
But there will be a lot of suspicion amongst boat owners and you will have to convince them that you have something to contribute (from your post it's apparently not sailing skill) and that you constitute no threat to them or their boat. Like they say, there's no such thing as a free lunch.
Good luck with that.
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"Two possibilities exist: Either we are alone in the Universe or we are not. Both are equally terrifying."
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