From the time we were only a city-state, we have worshipped His Holiness our Lord Lydien, Ruler over all waters, may His reign last forever and more. Just as our Lord is the most beloved god in the country and always One of the most influential—regardless of whether the current king chooses him as the main patron of the country—His Church is the most beloved and the most powerful.
The Church of the Water God was first built millennia ago to give thanks to our Lord for keeping the river flowing but not flooding and the irrigation of our fields abundant. And also—or rather mainly—to appease His anger.
The most basic way to do that is through prayers, however those can also be done without the presence of the church. Indeed, most houses in the capital and its region and many still in the rest of the country have an opening in their entrance hall to let rain in, and an impluvium, a pool to collect it. This invites our Lord in our home and demonstrate how much we cherish His presence. The members of the household gather there to pray in the morning before leaving the house, at night before going to bed, whenever it starts to rain, or whenever the weather is too dry. There are different types of prayers depending on what is supposed to be our Lord's mood, so that we can thank, beg, or appease him as appropriate. Although appeasing Him and debasing yourself is always appropriate.
What the Church is responsible for is giving longer and most powerful prayer in the name of the whole community. They can do this because they have trained to increase the output of magic in their voice while singing prayers. They are also intimately aware of our Lord's mood, thanks to constant observation of the sky, as well as direct interaction with Him. This allows them to be able to fine tune the content of their prayers in a way the rest of us cannot. And for this benefit, we all pay them a not unsubstantial tithe. An entirely voluntary one, of course. Just as living is voluntary.
In addition to daily prayers, the church organises a festival once a month and a bigger one four times a year. Those festivals are everyone favourites, whether our Lord is their main personal patron or not. In order to properly appreciate our Lord's power and generosity, all participants have to immerse themselves in water. This is done either in a nearby river or lake or in special pools attached to the water temples. While rich people have their own pools and can have a private celebration, it is not half as fun. During the public celebrations, participants swim and play in the water with each other, with regular interruptions for singing prayers, play in sport competitions, as well as eat food prepared by the temples and paid for by generous donors—we will absolutely not lord it over everyone else for months to come.
With so many people in the water at the same time, accidents are quick to happen. However, our Lord always pays attention to us during His festivals, and He will prevent any drowning. Generally. But you can be sure that anyone who actually drowns has offended Him in some way, and everyone will only say good riddance to them!
Anyone can join the Church, although not just anyone can rise up the ranks. Most people in the country will actually donate a bit of their time to the Church in order to win favour with our Lord—or the priests. Debtors can also see their debts bought back by the Church; they then become its indentured servants until they have been able to pay it back. Both of those groups of people carry out the most basic—and ingrate—work needed by water temples, such as taking care of the pools, cleaning the buildings and farming the fields owned by the Church.
Ordering them around are apprentices. Those are children who have been given up by their parents or dedicated at a young age to the Church. Once fully grown up and trained, they become priests whose role it is to take care of the apprentices and to carry out most of the prayers given to our Lord. The priests can them go up the hierarchy depending on their seniority, skills in their daily tasks, magical talents, and family's connections. Of course, the top of the Church is only filled with nobles. Commoners would not know how to behave in those elevated circles, anyway. As repeated many times, having them there would be a sure way to insult our Lord.
Being one of the leaders of the Church is very prestigious, and many noble patriarchs and matriarchs will attempt to become one, as well as educate their children to this end. However, being at the top of the hierarchy comes with a price...
While the Gods' Rules state that gods cannot possess or magically interfere with nobles without their permission, by dedicating themselves into a god's service they relinquish this protection. Other gods are also forbidden from directly interfering with each other's church, but becoming one god's favourite toy is already more than enough.
As most other gods, our Lord uses His priests to do His biddings through the country. And when ordering them around is not enough, they become vehicles for possession by Him. But few priests get that incredible honour or even that of talking directly with our Lord. Indeed, He mostly only bothers with the leaders of His Church. Yet, even among them, His favourites have a Brand to make them stand out. Contrary to other gods, our Lord is strong enough to be able to protect those He marks, and so the brand is a proper warning to other gods and mortals to stay out of the way of His agents.
Our Lord and His Church are not currently in an all-out-war against any other god—and thank them all for that! The water Church and the other major Churches of our country all give reluctant respect to each other. However, all those branded by the gods fight each other fiercely in the political arena, and there may sometimes be some slips into actual conflicts. This is especially the case during succession dispute such as what is happening right now.
While in theory the only role of the water Church is to pray and appease our Lord, in practice the Church is deeply involved in court politics. With many of the most powerful noble families leading the Church and other Churches lead by their rivals, any conflict can escalate quickly. However, it seems that most priests are quite careful with not falsely claiming that our Lord supports or disapproves of anything that He does not actually, as that never ends well for them. Or anyone else, for that matter. Of course, this does not prevent them from making many unsubtle insinuations...
For the rest of us, it's best to stay out of the way of our Lord and his Church alike. Give them the respect they think they are due, pray when you have to, and participate in festivals so that you do not stand out, but do not actually mix with those crowds. It is never worth it in the end.