Manuel Ortega posted a link above, but easy to miss. Here again:
http://www.mp-labs.com/sh2mods/sh3_uwfog.zip
You can also download this very promising looking mod from the same autor. The water mod is also included here. It´s still a little beta, but is a great improvement!
http://www.mp-labs.com/sh3mods/visibility_mod_v091.zip