4 new underground river biomes and improved generation are in my worldgen datapack Tectonic, check it out!

The datapack adds underground rivers to your world! They are located under where saddle valleys generate (so mostly around mountains). They have been designed so you (usually) can boat right through them and continue on in your adventures. There's two biomes for these rivers as well: the Lush River and the Dripstone River, variants of the Lush Caves and Dripstone Caves respectively. Dripstone River biomes even generate hanging lanterns!

Warning: This is incompatible with other worldgen packs, including my other tech demos. Do not ask for versions with Terralith compatibility, I will not be making them.