This is the glass covered version and looks nice when it's only 1 space deep, as the entire 2 block height looks full of air covered by glass:

But the same rails in deeper water look like the entire 2 blocks are full of water and makes it so you can't see much. Nowhere near as nice:

The basic rail would look a lot more aesthetically pleasing if the rail block was covered in water rather than air:

Other than that, only need a powered rail which I know you have planned, and this will be an excellent modpack addition ! I love underwater rail networks !

All of these have been or will be fixed with the soon to be releases 1.16.5/1.18 releases.
Unfortunatley i do not think i will have the time to get these fixes into the 1.12 version.

All of these are fixed for both the 1.16.5 version and the 1.18.1 version.