Australia doesn't just leave boats in the sea. They return people to their country of origin, take them to a third country, take them to Nauru or bring them temporarily to Australia.
They can only return people or take them to a third country with the country's permission.
By not processing claims and returning people to countries where they could be in danger, Australia are breaking the law. They also detain people indefinitely.
You're already aware of this because I've had this discussion with you several times.
We can't take people back to France because France won't accept them back.
Again you're talking about illegal immigrants rather than asylum seekers. Asylum seekers claim asylum in order to gain refugee status. Illegal immigrants get into a country to work and live there without a visa.
We do send economic migrants back, for example we used to get a lot of Albanians but deported them and rarely get any now.