With the new sewers and an emphasis on the dark ambience of it, I present four different shader packs you might be interested in making use of! Even though I've used the higher end of the Shaders for the screenshots below, even the less intensive versions are impressive and improve immersion greatly. All of these come with excellent customisation so you can tailor your surroundings however much you like.
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Chocapic13 - Very dark, if you crave that aesthetic. (Also got brilliant weather effects)
https://imgur.com/a/z6v4YMC
Robobo1221 - Probably the least intensive
https://imgur.com/a/csYcC2b
Sonic Ether's aka. SEUS - No sacrifice of vision for quality.
https://imgur.com/a/XVCNwpA
Sildurs Vibrant Shaders - Similar to SEUS but holds well alone
https://imgur.com/a/F8d5Jro
=-=-=-=-=-=-=-=-=-=-=-=
Chocapic13 - Very dark, if you crave that aesthetic. (Also got brilliant weather effects)
https://imgur.com/a/z6v4YMC
Robobo1221 - Probably the least intensive
https://imgur.com/a/csYcC2b
Sonic Ether's aka. SEUS - No sacrifice of vision for quality.
https://imgur.com/a/XVCNwpA
Sildurs Vibrant Shaders - Similar to SEUS but holds well alone
https://imgur.com/a/F8d5Jro