So watercooling doesn't necessarily mean 'quieter', all it does is have the capacity to move heat from the component to the air faster thus allowing more heat per second to be dissipated. As Pob pointed out earlier watercooling is great for overclocks for this reason. As for a 'quiet' PC, there isn't any difference in acoustic performance between a non-overclocked air-cooled and non-overclocked watercooled PC since they both feature the same fans. However the air-cooled will struggle with the extra heat outputted per second under heavy overclocking while the watercooled will be able to cope with this.