On Linux the OpenGL renderer is used by default, but the clouds/moon/sun conflict with the far clip fog. A good workaround is using the Vulkan renderer which may perform better but is less battle tested:
- Right-click Unturned in your Steam Library
- Select Properties... to open the General tab
- Find the Launch Options field
- Type "-force-vulkan" without quotes
Note some mods which were exported without Vulkan shaders will not be displayed properly. The majority of mods should have Vulkan shaders however.