You have to first flood just 1-deep area to make source blocks, and the blocks need to have 3 source blocks touching another block to turn that in to a source block <- Wrong info. So you can’t join two parallel water sources. Keep this in mind, not sure how much you’ve worked with water but the area in the screenshot won’t fill with source blocks unless you have a water source to the right aswell. <- also wrong info.
Also, when reflective water is below the default water level, the reflection shaders break and it looks really funky, see below, bottom right corner. This is just one block from the base water level, the lower you go, the worse it gets.
These are some of the things I think people should know before doing the huge investment in water-builds.