Try this:
echo CHtml::dropDownList('island_id',$island,$locationsList,
array('ajax'=>array('type'=>'POST',
'url'=>CController::createUrl('supplierHead/getRegions'),
'update'=>'#region_id',),
'id'=>'island_id',
'empty'=>'Choose Island',
'onchange'=>'$("#province_id,#region_id").children().html("<option value=''>Choose Province/region</option>")'
));