If you want a balcony cabin then it has to be deck 9/10/11, best to select one mid ships. The widest selction is on Deck 10 Sapphire.
http://www.islandcruises.com/downloads/5.pdf is the page with the ships deck plans on it.
Why is it best to select a cabin mid ships?
Mainly two reasons the most stable part of the ship as far as movement is concerned and it means you are usually closer to most of the amenities.