This is the underwater cathedral in my underwater city, Aquain.  Like other parts of the city, the church was made in a realistic way.  I followed the standard cathedral structure with a nave, transcept, aisles, etc.  Baptismal font, altar, side chapels and all the fixins.  I also put another floor in the basement with church offices, classrooms and a rec center.  Eventually I'd like to put in crypts as well. 

St. Brendan is the patron saint of divers, mariners and sailors, so he was a good choice for this church.  This was made in survival mode.

To see the rest of the city, go here:
http://www.planetminecraft.com/project/aquain-a-realisticly-planned-underwater-city/