65.78% of the water is is frozen at the poles.
Some countries trash rivers which sometimes can end in the ocean because they are connected.
Cambodia and Vietnam are not island nations: they are not placed on islands, but on the Indochina Peninsula in Asia - so they can't be the correct answer
Indonesia is an island nation, but it's a presidential republic, not a monarchy.
The correct answer is Brunei
Rainwater is the main solution. There would need to be higher technology, to make that happen.