Float flying certainly is specialised, however I think the available supply of experienced foreign float pilots might dry up (no pun intended) whether 457 visas are possible or not. Canada and the US probably have the greatest supply of float pilots, however those pilots now have plenty of employment options close to home, in both landplanes and seaplanes.
Whilst I am not very familiar with Australian float operators, those operating elsewhere in the country seem to find a way to keep their aircraft flying. Is this just a Sydney thing?