For starters, you don't have an ATPL with 250 hours.
All the commercial float/sea plane operators I know require more hours and experience than 250 hours.
For a float endorsement, you need to find a school or company with authority from the regulators that do them. I doubt Oxford (OAA) do them.
The company I worked for in the Kimberley (Aus) used to do them for suitably keen pilots but they're out of business now.
Re working in Australia, you need to fulfil the visa requirements to work there.
I've worked with guys who flew the float Twotters in the Maldives, he had thousands of hours before he got a job.