From A Wiki of Ice and Fire
Ironmen become Drowned Men by being submerged in salt water a second time in earnest and brought back to life with a crude form of cardiopulmonary resuscitation. Not all men are successfully revived, however.
They wear roughspun robes of mottled green, grey, and blue, the colors of the Drowned God. They carry driftwood cudgels to show their devotion in battle, and skins of saltwater to perform ritual anointment as well as to sate their thirst.
While Priests of the Drowned God, must not shed the blood of ironborn, they have no such reservation about other methods of killing, such as drowning.