Floor boards was badly rusted, especially on drivers side. Covered under Nissan general through rust warranty. Afterwords, I sanded down drivers side floor board inside car, and painted on POR15. Now it does not matter if it gets a little moist under there!
As for POR15. Its a serious restorer product available from manufacturer por15.com (or in Canada - porcan.com) - no resellers. If you decide to go with that, I can give you some starting tips that can save lots of time. Let me know.
As for where the water comes from... I don't really know. I put in some plastic bags under carpet for a while, on top of floor board. Water was clear, no sand nor salt was appearant (roads are salted in winter season here). This makes me believe that it comes from 3 sources in this order:
1) Wet left drivers feet.
3) leaky or missing floorboard plug
As for 1) Water is not catched by the removable floor mat. Water will then be filtrated into clear water while going through floorboard mat. In 2004 (My guess), Altima came with a plastic dead pedal. It keeps foot further back, which should keep water away from floorboard mat. This may have been Nissan attempt to fix problem.
As for 2) Condensation. In the winter, a leak in a heater duct routed under floorboard carpet may let warm air condensate on cold floor. The heater duct is a detachable multi-piece unit, which may leak in its first joint.