Queensland Police told 9news.com.au the crash occurred just after 12pm AEST at Jumpinpin when the seaplane careered into the water.
Other people on boats in the water watched on in panic or rushed to help after the seaplane crashed.
The flight appeared to have a wobbly take-off, before coasting along the water and stalls before cartwheeling into the sea.
There were only two people on board, a man and a woman.
They got out without injury but were taken to the mainland to be assessed by paramedics.
“No persons trapped or injured so it’s a good outcome,” a Queensland Fire and Emergency Service spokesperson said.
“You never really know what can happen, issues, maybe some fuel leakage no dramas in that regard.”
Authorities pulled the plane from the water and the Australian Transport Safety Bureau is now investigating.
Public holiday crowds had packed out the waterways and emergency crews say it was lucky the plane didn’t hit any other boats.