There doesn't need to be much below the water, with the likes of QM2, Shane. Since she's only carrying passengers. The likes of the Berge Stahl, an Iron Ore carrier, Bulk Carriers and the likes, need to have a lot under the waterline. As well as other vessels like Tugs etc.
Also, they would also want to keep their draught to a minimum, since they would want access to as many ports as possible, as well as being as agile as possible.
Jack.