I don't know where you live, but maybe you bring in to much snow when you get in. Shake and clap your boots before you get them inside.
Try to remove the carpet. There may be a hole, crack or something that let the water in.
Look under, any rust spot? Does it goes through the metal?
In those case, you need to weld a metal plate to plug any hole or crack.
I don't know for your model, but several cars have drain holes under the carpet, but they often are fitted with rubber plugs. If that's the case, remove those plugs. The water will still get in, but will be allowed to drain off.
Check the air conditioner if you have one. It may have a leak.
Check the air vents. They must be dry. If not, try to find from where that water comes in and why it don't drain off by the drains that should be there. If could be just a mater of cleaning them.
Whatever the origin of the water, you will need to replace the carpet and under-carpet to prevent/remove harmfull, and malodorous, mold.
Electro, February 2013