This mod was originally made for Modjam 5 (2018.1) but has since been updated!

It adds 2 new rails to the game:

Basic Underwater Rail (Can be placed underwater, but does not stop the player from drowning)
Advanced Underwater Rail (Can also be placed underwater and it stops the player from drowning while in a minecart on the rail, as well as disabling the underwater fog)

NOTE: In 1.18.1 there is only advanced underwater rails (named just Underwater rails), this is due to vanilla rails now being placable under water in 1.18.1.

NOTE 2.3.0 and up for Forge requires Architectury, and any version for Fabric also requires Architectury.

The Fabric version currently does not disable/change underwater fog, unlike the Forge version. I hope to get this added eventually.

These all come in 4 variants,

Standard rails
Powered rails
Detector rails
Activator rails

Currently the only supported MC versions are 1.16.5 and 1.18.1, with 1.18.1 taking priority over 1.16.5.

the 1.12 version is unlikely to get further updates! Forge is generally prioritized over the Fabric due to me being more familiar with Forge than fabric.

Recipes

Basic Underwater Rails (only 1.16.5 and older):

Shaped Recipe:

Shapeless Recipe:

Advanced Underwater Rail:

NOTE: For 1.18.1 the recipe is the same, but you need to use the vanilla rails instead of the now non-existant basic underwater rails.

Known issues

Advanced underwater rails are currently only modeled for standard and curved orientations, meaning that when used in a slope the default raised rail minecraft model is used(still functions as an underwater rail though), i hope to get this fixed but i am held back by my lackluster modeling skills, help is appreciated here!
Underwater rails sometimes do not render properly underwater, this seems to be a vanilla issue that was fixed/improved with 1.17/1.18. So it is unlikeley to be fixed.