From A Wiki of Ice and Fire
The Drowned God, also known as He Who Dwells Beneath the Waves, is a sea deity worshiped solely by the ironborn of the Iron Islands in Westeros. The religion of the Drowned God is old, dating back to before the Andal Invasion. The Andal invaders of the Iron Islands converted to the local religion rather than supplant it with the Seven as they did in the south of Westeros. The Drowned God religion supports the ironmen's naval, pirate culture.
Like the ironborn, the Drowned God is a harsh deity and goes hand in hand with the Old Way. It is said the Drowned God made the ironborn to reave and rape, to carve out kingdoms and to make their names known in fire and blood and song. The Drowned God himself is believed to have brought flame from the sea and sailed the world with fire and sword. The Drowned God's eternal enemy, the Storm God, resides in a hall within the clouds and ravens are his creatures. It is said the two deities have been in conflict for millennia and the sea roils in anger when they engage in battle. However, much like the Drowned God, no one aside from the ironborn believe in the Storm God.
When an ironman drowns, it is said that the Drowned God needed a strong oarsman, and the refrain "What is dead may never die" is used. It is believed he will be feasted in the Drowned God’s watery halls, his every want satisfied by mermaids.
Drowning and resurrection feature prominently in the prayers and rituals of the Drowned God religion. Drowning is the traditional method of execution for the ironmen, but it is also considered a holy act, and the most faithful have no fear of it. Newborn are "drowned" shortly after birth, being submerged into or anointed with saltwater. This is done as part of rites of the god, committing their bodies to the sea, so when they die they may find the Drowned God's halls.
During the anointment ritual, the priest has a person kneel. Using his skin of sea water, he pours a stream of it upon the person's head. As he does this he intones:
|“||Priest: "Let <person> your servant be born again from the sea, as you were. Bless him with salt, bless him with stone, bless him with steel."
Response: "What is dead may never die."
Priest: "What is dead may never die, but rises again, harder and stronger."
Clergymen, called Drowned Men, are drowned a second time in earnest and brought back to life with a crude form of cardiopulmonary resuscitation. Not all men are successfully revived, however. Drowned Men wear roughspun robes of mottled green, grey, and blue, the colours of the Drowned God. They carry driftwood cudgels to show their devotion in battle, as well as skins of saltwater to perform ritual anointment and occasionally drink from to strengthen their faith.
While priests of the Drowned God must not shed the blood of ironborn, they have no such reservation about other methods such as drowning.