Bit of a complexity over in the US with their low flying rules too. If I'm not mistaken, you can fly low if it's not a safety risk?
Agreed 100% that to be low flying at even 45 degrees as a 20 something hour sport pilot feels like it's asking for trouble. In Aus I'm guessing you'd need the initial 20 hours (usually more), a float rating but presumably no low level rating assuming you're only taking off and landing on the water - not mucking around just above it.
However.... you've been able to buy an LSA seaplane/floatplane since a long time back now, so nothing's changed legally, it'a just all in the marketing. The same thing happened with wind surfers, kite boards and Jetskis - people were killed during all of those coming on scene.