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President Mohamed Nasheed and his cabinet signed a document calling for global cuts in carbon emissions.
Ministers spent half an hour on the sea bed, communicating with white boards and hand signals.
The president said the UN climate change conference in Copenhagen this December cannot be allowed to fail.
At a later press conference while still in the water, President Nasheed was asked what would happen if the summit fails. "We are going to die," he replied.
The Maldives stand an average of 2.1 metres (7ft) above sea level, and the government says they face being wiped out if oceans rise.
"We're now actually trying to send our message, let the world know what is happening, and what will happen to the Maldives if climate change is not checked," President Nasheed said.
"If the Maldives cannot be saved today we do not feel that there is much of a chance for the rest of the world," he added.
Click the title to watch a video
A video and artcle about how sea level rises have affected the Maldives
A video and article on how the Maldives have tried to become carbon neutral