something to be careful on with the large trailer boats from US is their beam, max unrestricted width in Australia
as I recall
is 2.5m, whilst in US it is 2.6m. Introduces issues of towing an overdimensional load.
Also be mindful that boats of the type mentioned in David M's listcome from the land that invented the expression "there is no substitute for cubic inches". They are all heavy boats requiring alot of power, and hence petrol to pus them along.
The trailer issues probably relate to compliance with some of the weird Australian Design Rules, like having to weld safety
chains to the frame (which weakens them), tow ball coupling type, lights and brakes. Many US trailers have drum brakes which last about 5min after salt water
At the end of the day you pobably would spend the same money to import
as you would for something bought in Australia