I would even assume that in some cultures becoming a drown is an honor.
I think the Fellowship and the Overlords are the least likely to have a significant agendas for empires that adopt them. The overlords can be beign, even beautiful. I always liked Xeunay's writing for the overlords because he always seemed to capture this.
Crazed madmen may be their most obvious traits from an army/empire perspective. But I imagine their actual worship to be much different.
I would imagine their temple sanctuary to be an organic shaped chamber with smooth blue and green glass tiles all over the walls. Like being inside a jeweled kidney. Water would run into the room through the altar and then through the chamber in fat grooves cut in the floor. This single flow of water would go throughout the temple, it would be the same that passed through each of the dreamers pools in more private chambers. It would also have a slight halluncinagetic effect on those who drank it, so that worshippers would occasionally see visions and hear messages while worshipping.
Because of the water the air would be thick and moist and there wouldn't be any real organized cermonies. Just faithful dreaming, praying, or talking to each other throughout the temple. In general the atmosphere would be quiet, but occasionally screams of rapture or alarm would ring out. Most would simply ignore them.
Some would be simply faithful decent people (this is where a lot of the assumptions go wrong, they think to specifically about what a given religion is) coming to ask for safe travel on their next naval voyage. They may annoint themselves with the water, pray for themselves and their love ones and leave. Some may drink and have no or a very minor reaction. Others may live in the temple, completely lost in the visions.
There are good people that worship the overlords, there are bad people that worship Junil. But as a state religion the overlords are going to require a certain loose morality to handle the things that go on in the temples. They arent very compassionate so its hard to imagine a good empire that worships them, but they dont have a dark agenda for their empire so it would be easy enough to be neutral with them.