Navigate to Options > Resource Packs > Open Resource Pack Folder. Drag and drop the downloaded ZIP file into that folder.
Underwater Clarity: Remove the murky darkness of oceans and rivers. fullbright texture pack 1.12.2 no optifine
Compatibility: Certain technical modpacks or specific Forge versions in 1.12.2 can crash when Optifine is added. Navigate to Options > Resource Packs > Open