Without "Slidur's Enhanced Default" Shader (Iris Shaders):
All fog works perfectly as expected. All configuration options work perfectly as expected.

With "Slidur's Enhanced Default" Shader (Iris Shaders):
Surface fog works perfectly as expected, but underwater fog is reverted to its vanilla behaviour. Configuration options for water don't do anything.

NOTE:
Slidur's Enhanced Default doesn't make any changes (or any visible ones, at least) to fog, so it seems strange that it isn't fully compatible by default, and even stranger that It's only water fog that's broken. Also, my apologies if this isn't fixable on your end. I don't mean to waste your time.